What age do you mean that by though. A 13 year old (no offence to anyone who is 13 by the way) might take it the wrong way and going round like it. Many people my age do the same, without the violence but the genral attitude of it. Surely its about maturity, ok the age is there are a reason but I can play these games and not going round thinging im a gangster and wanting to shoot people, maybe somone can't but I still think a lot of people my age who can play the game no problem. Its the ones who can't at my age that I feel the age is right.
Another thing the age isn't going to stop people playing it as I said above, its not just parents buying people the game for the younger people, might do an experiment and see if I can buy the game at 16 from a shop, would be intressting, though probably won't do it.