I'm 14, and I've been playing since I was 5. I really loved cars then and my dad bought me GTA III so I could drive, and I thought it was really cool. A couple of months later I found a baseball bat, then started doing missions and now I'm here.
The game hasn't given me any desire to kill anyone, steal cars or anything, but I will admit that it's desensitized me to violence. I think that goes for pretty much anyone who has played more than one GTA, but still. Anyway, I'm mature enough to play and I think that anyone who is mature enough to play can play. It's not like the second you hit 17, you suddenly have to urge to kill anyone anymore if you did previously. If you have mental issues, then the game might set you off. Doesn't matter how old you are.