United States
Under Federal Law,
Airsoft guns are not classified as firearms subject to the Gun Control Act. (However, In almost every major city and population center the definition of a firearm within their respected ordinances includes propulsion by spring or compressed air, thus subject to applicable laws.)
A 6 mm minimum orange tip must be present on the barrel end of the airsoft gun to identify it as such for any commercial sales or transport. Once you own the airsoft gun, you may remove the orange tip in most areas, but local and state laws do vary.
Airsoft guns' trademarks must be removed where the manufacturer does not have an existing license agreement with the "Real Steel" gun's manufacturer in the US. (Example: Classic Army has a licensing agreement with Arsenal Arms, so the trademarks can stay on imported replicas of Arsenal's SLR 105 A1. In practice enforcement is hit or miss. You might get an "unlicensed" gun through customs with Trademarks intact, while a Licensed gun might be held in Customs by an "underinformed" customs agent.)
In addition, the similarity between genuine firearms and airsoft replicas is close enough to provoke interaction with local law enforcement personnel if an airsoft gun is carried openly in public.
If someone were to, for example, attempt a robbery with an airsoft gun, they would be charged as if the airsoft gun were a real firearm.
New York City requires that all realistic toy or imitation firearm be made of clear or brightly colored plastics; furthermore, New York City makes possession of any air pistol or air rifle or similar instrument in which the propelling force is a spring or air, unlawful without a license. See New York City Administrative Code § 10-131(b) and New York City Administrative Code § 10-131(g)(1)(a). This has the effect of creating a total ban on airsoft guns since few manufacturers make such guns, and fewer players desire them. The rest of New York State is unaffected by these laws, and there are no state regulations limiting or prohibiting airsoft.
Michigan restricts the purchase of these guns so that they may only be purchased from a licensed retailer.

Canada
A gun is classified as a firearm, if its velocity is high enough for a fired projectile to penetrate the eye of a pig (grievous bodily harm). Using this test, it has been determined by the RCMP, that Airsoft guns firing under 407 ft/s (124 m/s) are not firearms.
However, Airsoft guns that closely resemble Real firearms are classified as Replica firearms and can only be imported by companies possessing a Business Firearms License. It is unlawful to sell or transfer Replica Firearms without this license.
Airsoft guns firing between 407 ft/s and 500 ft/s (124 to 152 m/s), are classified as Uncontrolled Firearms and do not require licensing. Above 500 ft/s and 5.7 joules, Airsoft guns are considered Controlled Firearms and must be registered.
No legal distinction is made between airsoft and true firearms when they are used for the purposes of crime.
In Ontario the minimum age to purchase airsoft is 18. Childern under 18 must be supervised by someone over 18.
You cannot import any airsoft guns firing under 407 fps into Canada without a licence.

Australia
In Australia airsoft is illegal for most practical purposes. Australian law classes Air and BB guns as firearms, and acquiring a licence to possess (e.g.) a pistol requires a rigorous and laborious process of police background checks. You must also have a legitimate reason and be able to prove that reason before you are allowed possession of a firearm, regardless if either airsoft or live-fire. Anyone found in Australia possessing an unlicensed airsoft pistol faces the same charge as a person who unlawfully possesses an actual firearm.

Airsoft Australia is aiming to get airsoft redefined as a sport and allow it to be legally accepted. This will only be possible by restrictions on how replicas can be obtained (Over 18 and an airsoft specific licence (is hoped) to be required of the player), for protection of the sport in general and others.

Austria
Airsoft guns and pistols are allowed up to 0.8 joule.
All users that are at least 14 years old do not need their parent's permission.
Airsoft guns and pistols more than 0.8 joule can be purchased in specialised weapon shops only and all users have to be at least 18 years old

Belgium
Airsoft guns and pistols can only be bought at officially licensed dealers, who carry a government permit to import and sell firearms.

Czech Republic
Trade with them is regulated under the weapon law and all airsoft guns must be authorized by the Czech Gun and Ammunition Testing House before usage.
Airsoft guns have the same status as real weapons, described as sport weapons.
The use of airsoft guns is allowed for players that are least 18 years old. If children of at least 15 years want to play, an adult must supervise the game.
Airsoft guns may not have an energy greater than 16 joules.
The use of laser sights and illunimators is forbidden.
Usage and open carrying of air soft guns in public places.

Denmark
Airsoft guns are mentioned in the Danish "Våbenlov" (Arms control legislation).

You have to be at least 18 years old to buy, hand over, possess and use airsoft guns.
A firearms certificate is not required.

Finland
Visible transportation of replica firearms in public areas is forbidden. All replica firearms must be covered with something, for example, a weapon case.
Land owner's permission is needed to play airsoft in any area.
Minors (under the age of 18) are able to purchase airsoft guns only with written permission from their parents.
This has caused children even as young as 11-12 years old to get airsoft guns. There has been some problems with kids playing with a "toy" that can be somewhat dangerous. No big incidents have occurred, but this is very annoying to the real airsoft players who want to get airsoft's rather bad reputation higher than it currently is in Finland.

Germany
Airsoft guns under 0.5 joule are considered toy guns and can be freely sold to all persons above 3 years of age. Distributors agreed to raise the limit to least 14 years of age.
All airsoft guns above 0.5 joule and below 7.5 joule must be bolt-action or semiautomatic only and can only be sold to people 18 years or older. These are considered "free" firearms, as a result:
Sales of guns of more than 0.5 joule are allowed only in weapon shops.
Guns must be marked with the trader's weapon abbreviation and a F-in-a-pentagon mark as well as the airsoft gun caliber (such as 6 mm BB).
Target illuminating devices and lasers may not be attached to guns but are legal otherwise. For example: possession of a flashlight is allowed, even shooting with the flashlight in one hand and the gun in the other; but attaching it via mount ring to the rail system of a gun is not. Devices made specifically for the purpose of being attached to a gun (like certain flashlights with integrated foregrip for mil-spec rail) are prohibited.
While the possession of airsoft guns is allowed, the actual use in a game is (at least) hotly debated. For sure, most players using guns with more than 0.5 joule muzzle energy leave Germany to play in countries like France, Belgium, Denmark or the Czech Republic.
More information can be found at Airsoft FAQ on laws in Germany, which covers more complicated issues like the "Kleiner Waffenschein", issues with the OWiG §118 in Bavaria and a definition of the term "combat shooting".

Hong Kong
The kinetic energy of the airsoft gun must be lower than 2 joule
Only allow to play airsoft in private area and non-country park area.
Cannot show the airsoft guns part in public area.
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Indonesia
In Indonesia, there are no strict rules about it, and still under consideration by the government as if to classify airsoft guns as "toys" or almost equal to real guns. However, airsoft were first brought to Indonesia circa 2000 - 2001, and the biggest retailer being Toy Saurus at Senayan Trade Centre 2nd floor, Asia Afrika Street - Central Jakarta. Fortunately, the founders of Indonesian airsoft communities put some restrictions on this game. For example, airsoft players are prohibited to upgrade their gun to above 100m/s, or they'll be rejected from the community. And the one who wants to buy airsoftgun, must be at least 19 years old and know the regulation and rules about the airsoft gun. The biggest airsoft community base are on Jakarta (Bravo Brigade), Bandung (Vertex Squad), and Jogjakarta (Ground Zero).

Ireland
Airsoft was made legal in Ireland after the 2006 Firearms act. Airsoft guns must be 1 Joule or less to qualify as a toy.

Israel
Airsoft guns are classified as "dangerous toys" whose import, manufacture, sale and possession are banned. This law is not very well enforced, however, and it is possible to find retailers who import MPEG level Airsoft guns. Legal issues notwithstanding, the sale of Airsoft guns in Israel obey the rules of demand and supply.

Italy
Airsoft guns and pistols are allowed a velocity below 100 m/s (328 ft/s) i.e. equivalent to 1 joule: under the law, airsoft guns are not classified as firearms. You can buy and sell it both from stores and from another private citizen.

Orange tips must be present on the barrel ends of the airsoft gun when they are imported and sold by a store. Once you own the airsoft gun, you may remove the orange tip; however, the similarity between genuine firearms and airsoft replicas is close enough to provoke interaction with law enforcement personnel if an airsoft gun is mistaken for its real counterpart. Airsoft used to commit a crime is treated as if you had the real gun, assault weapons carry an extra mandatory sentence in addition to the regular punishment for the crime committed.
The minimum age to purchase airsoft and/or use it during a regular match is 18, or 14 if accompanied by a parent or legal tutor.
Usage and open carrying of air soft guns in public places is forbidden. You can play in a private property away from public sight, or in a well-delimited private or state property after having asked the local authorities for a limited-time permit (usually from 6 to 48 hours), and having alerted the local police command, to avoid alarmed citizens calling for emergency.

Japan
Legal requirements are set on airsoft model manufacturers to prevent any possibility of a replica weapon being converted into an actual firearm.
Standards include (but are not limited to) use of low-melting point metals and non-ballistic plastics in structural components and incompatibility of mechanical components with actual firearm components and mechanisms.
The overall litmus test used by the Japanese National Police Authority is whether the replica weapon can be made to chamber and fire an actual round of ammunition.
These standards have proven successful within Japan, as it has been found that criminal elements discovered that it is significantly easier to purchase an actual illegal weapon in comparison to modifying a comparatively fragile replica into a functional firearm.
Due to this reality, most crimes involving a threat of physical violence are often perpetrated with edged weapons, as firearms seen in public are (by default) believed to be toys by the public at large.

Luxembourg
All airsoft guns are treated under the national weapon law and demand a personal user certificate.

The Netherlands
The law places full restrictions on Airsoft Weapons, rendering possession illegal. When issued a permit, this is legal, as (realistic) airsoft weapons fall in Category I, requiring a 'permit' (ontheffing). Non-realistic 'air-pressure' weapons fall in Catergory IV, and may only be 'carried', not fired, and require a 'permit' (onheffing). When enquiring more information please contact the Dutch airsoft association: www.nabv.nl

Norway
The Arms control legislation (Våpenforskrift) requires:

One to be at least 18 years old to buy, hand over, possess and use airsoft guns.
A firearms certificate is not required.

Philippines
Organized airsoft started in 1985, and interest in the hobby had gone up and down, several times over the past 20 years. The airsoft gaming community is highly secretive and underground, but has seen a tremendous surge of newbies in the last 2 years, owing to the advent of cheap China-made airsoft. Airsoft teams are often very clannish, with a number of groups claiming representation, to a certain extent, of the local airsoft community, organizing and coordinating between local teams, especially during big events where hundreds of players from teams all over the country converge on selected venues for friendly tournaments. Letter of Instruction 1264, signed in 1982, bans the importation, sale and public display of gun replicas, but purchase of airsoft guns and the movement of airsoft players are largely untouched by the government, with a few confiscated shipments only marring that record. No direct regulations have been placed on the airsoft community, and players of all ages are welcomed to play. In the Philippines, a firearm is considered lethal and illegal if it fires a projectile larger than 5.5mm in diameter (most BB's fire pellets 6mm), however, local media has suggested that airsofting will soon be considered officially legal provided there are a few exceptions like the proposed ordinance of repainting the replica gun to make it look less realistic and more distinguishable from an authentic firearm (similar to laws in the United States). However given the structure of the Philippine government and their method of operation, such a ratification may take several years to be processed. Until then, most airsoft shops in the Philippines are discreet and well hidden from the public, often located in unmarked buildings and difficult to find shops where their wereabouts are only spread via "word of mouth" or by the internet.

Poland
Airsoft guns fall into the same category as paintball guns and smoothbore air-powered weapons up to 17 Joules and are available to people over 18 years of age, registration of any sort is not required. This, however, is not strictly enforced and many cheap spring replicas may be found in toy shops (due to common practice of labelling them as "toys"). Generally, the police considers airsoft replicas toys rather than "non-lethal weapons". The Polish airsoft community has formulated "Airsoft Rules", an unofficial set of rules regarding airsoft as a whole. While they are not enforced in any specific way, abiding "Airsoft Rules" is a sign of "playing fair" and belonging to the community. Excerpts from "Airsoft Rules":

Eye protection must be worn at all times during the game.
Brandishing replicas in public places is not allowed. Doing so may lead to ejection from the community.
Local law enforcement (police, Forest Guard etc.) must be informed earlier about every airsoft game taking place in the area.
Players between 16 and 18 years of age are able to participate in airsoft games only with written permission from their parents.

Portugal
Airsoft is legal in Portugal under the name of Softair. Softair falls into a specific category designated as "Arma de softair" or in english "softair gun". According to the new guns and ammunitions law (DR - Lei n.º5/2006 de 23 de Fevereiro - Regime Jurídico das armas e suas munições) some of the main excerpts are:

Any softair gun must be totally or partially painted in fluorescent red or yellow colour;
Maximum energy level at muzzle exit must not exceed 1,3 Joules (or 374 fps);
Softair gun purchase is limited to:
Minimum age of 18;
Only for sport practice;
Buyer must be registered in a softair federation;
Softair players/gun owners must possess Public Liability insurance;
Other special limitations may apply to softair gunsmiths and players.
This information is an excerpt of the law, for further information refer to full document (DR - Lei n.º5/2006).

Romania
Law nr. 295 from 2004 (Regimul Armelor si Munitiilor) regulates all use of weapons and associated ammunition:

The law is quite unclear (in what concerns airsoft weapons) as to whether this kind of weapon classifies as "non-lethal weapon" or "toy".
The law regulates the use of air-powered weapons (e.g. sport/competition use, that use a metal projectile) under "non-lethal" category and solely requires that you (1) are at least 18 years old and (2) register your weapon at the police precinct nearest from your location.
The law specifies that usage of night vision (infrared) or laser aiming devices designed for military use is completely restricted to members of the army and associated entities even if the aiming device is used on a lower-restriction category weapon (e.g. such as on an airsoft gun). The law, however, does not restrict in any way the use of aiming devices not designed for military use.
The law specifies that, should you attempt to use a non-lethal or replica gun to perform (or attempt to perform) armed robbery, you shall be prosecuted as if a real gun was used.

Slovenia
One has to be at least 18 years to buy airsoft guns.
If the velocity of an airsoft gun is below 100 m/s (328 ft/s) i.e. equivalent to 1 joule, it is considered to be a toy.
If the velocity is higher than 100 m/s (328 ft/s), the airsoft gun is classified as a section D weapon in the Firearms control legislation of Slovenia. Additionally Air Soft Clubs and National Shooters Association in Slovenia recommend that airsoft gun velocities should not be above 100 m/s (1 J).

Spain
Airsoft guns are regarded as low power weapons and AEGs must have:

Semiautomatic rifles and pistols: less than 24.4 joules
Other automatic electric guns: less than 1.00 joules and local police validation.

Sweden
One must be at least 18 years old to buy airsoft guns. Players in Sweden are often very protective about this, and do not like players under 18.

Switzerland
Airsoft guns are not considered as subject to the weapon legislation and no permission is necessary.
Please note that all kind of laser sights are forbidden.

United Kingdom
There are currently certain restrictions on the possession of airsoft replicas, which came in with the introduction of the ASBA (Anti-Social Behaviour Act 2003) Amendments, which prohibit the possession of any firearms replica in a public place without good cause. The prohibition of self-contained gas cartridge weapons similar to that made by Brocock can arguably apply to Moscarts and BB-Shower grenade systems, however a formal case precedent has yet to be set. The Violent Crime Reduction Bill, now under debate, may prohibit the sale, manufacture and import of any replica weapon, including those used for Airsoft. This would mean the eventual end of the sport in the UK, as guns break and could not be legally replaced. The Airsoft community in the UK and other countries have started a public campaign in order to try and save Airsoft there. Due to the VCR bill, British Airsoft players, retailers and sites have had to look to the future by forming associations, the first and prior to VCR was UK Airsoft Sites Governing Body UKASGB, followed by the retailers UK Airsoft Retailers Association UKARA, with the first official players body for the South West region UKSAPA, latest to be formed is the Association of British Airsoft ABA which is currently along with UKASGB in talks with the Government with regards to gaining an exemption for the sport. The ABA are asking people to show their support by filling in a Supporters Form and those people who are a part of the sport to register at the site in case membership is required by the government in the future to enable Airsofters to take part in their sport.