Because 18+ is a nice and neat number that businesses tend to use. Don't look at it too closely though. If the 17 is mature enough, "Please wait one more year" should be all the reasoning he needs.
There is no one universial age of majority in Canada, it's set by the provinces and is either 18 or 19. Here you find the liability issue.
Age of criminal responsibility in Canada is 12. Here's your culpability aspect.
18 is the age to vote, but that's hardly indicative of anything.
You can get your learner's licence around 16 in most provinces, and as low as 14 in some. Even with parental supervision, it's vastly far more dangerous situation than airsoft.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."