Kids around the age of 14 used to be allowed to purchase "M" rated games. I do not see any reason why this was banned. I am sure that these kids would have consulted their parents before going to a store (because, I'm assuming that you would need permission to go out anyway) and buying an "M" rated game.
I used to allow my son to go into the game store they have in our local mall and buy any game he wanted for his birthdays. Now, when I'm getting his other presents, he comes over to me and says "they won't let me get this game because it's 'M' ". It's unfair to all-of-a-sudden take a right away from our children, that we allowed them as parents PERSONALLY, to go and get a game without us being there.