You can buy one anytime theres no law preventing you from doing so, heck you could probably walk into a store if you were 8 and buy one. THAT BEING SAID. Whether they're going to sell to you or not is a different story. Most stores have policies of selling only to people 18+. The only place where it's really illegal is Ontario due to a provincial bylaw (18+ to purchase an air gun).
Also SOME and only SOME clubs/teams will allow 16+ with parental waiver (eg. I know JOC, Op-For, and BCAC allow 16+ with waiver). It's a general rule of thumb that once you're 18+ you're good to go anywhere.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE