Due to the nature of airsoft guns...specifically the fact that they look and mimic with precise detail the appearance of real fire arms. It just makes more sense to to ensure that those that handle them are at least the legal age of majority so that they and only they can be held responsible for their actions.
It looks like a real gun, if a person causes shit with it they get in trouble and not their parents/legal guardians.
18+ is only an ASC thing, each province has their own mindset and rules in regards to age restrictions.
"Its only a little bit on fire"