He contacted you through ASC? or this was from Craigslist or some other site?
ASC only controls ASC nothing more only some areas have a law about 18+ most of Canada does not. Even still I would simple not sell to him you do not have to tell him it's law or anything it's your item to sell to whom ever you want and if that is 18+ then that is his problem not yours. but what you do outside of ASC is up to you.