You really should have posted this in the Newbie Tank.
One of the short answers is that under Canadian law airsoft guns are 'unregulated firearms'. That means sale and transfer of them is restricted to people who are 18+.
Putting the restriction on the forums is just a very simple, and sensible way for Airsoft Canada to avoid a whole hell of a lot of liability in a case where unregulated firearms were found to be sold to minors.
Good luck with the thread.