If you were 16 and got an M-rated game, you probably look over 18 and he didn't think to ask.
The GameStop policy is very strict. If you get caught selling an M game to a minor, you AND the store manager get fired, even if the store manager isn't even there (because the logic is that the store manager should have trained you not to do that).
I guess the key phrase there is "If you get caught." Unless a parent complains or it's a secret shopper, you might not necessarily get in trouble for selling an M rated game to a minor. I don't know. I don't intend to find out. I always ask for ID on M-games unless they look over 30. When kids bug me about it I tell them that unless they can pay my salary I'm not going to do anything for them that will get me fired.