Agreed. If you know your limits and you're disciplined enough (like a martial artist, for example), you can play GTA, even if you're below 17. In fact, I've put GTA into practical use sometimes, using it as a resource whenever my seven-year old cousin asks me to draw something like a car for her projects...
Honestly, I don't think people that are 'disciplined' enough are the only ones that should be able to play GTA... I think the people that blame killing people on GTA are metally unstable and fucking stupid, and it's just some stupid ass excuse they make.
It's easy to play this game, and it's not that evil of a game, and you don't go kill people because you saw it on the game. Plain and simple.