Well, I guess I have to come in here again as I am both a parent and over 18 :)
I can only state from experience of my son, but as I already said he is totally non-violent in nature despite playing GTA games. However, he chooses not to play Doom3 or others of that nature. In fact he has no interest and has never even asked to play them.
He also plays Harvest Moon, Pokemon, Yu Ghi Oh and Monopoly (he has several versions :). He likes football (soccer for you lot across the pond :) ) and watching Nickelodeon. Today he was the narrator for the school carol concert and did a lovely job. :)
I feel he is pretty well balanced, although I am aware that not all kids are the same and my judgement is biased.