Airsoft is a sport and a hobby.
It's says only 18 because those rules are from the manufactures.
The labels also say check the area where you're from and to check the law.
It's different for most provinces and countries. Most is 18 to purchase.
The reason why the number 18 appears on all labels is because it is by majority that most countries allow over 18 to purchase AND to posses one.
In Canada, there is no law that states an age for those who cannot posses an airsoft gun.
Reading labels from manufactures that states 18+ is no reason why the sport and hobby of airsoft is 18+.