My son is five and has been gaming since he could hold a controller. He first took an interest in getting in and out of cars in GTA and since then has become just as keen a gamer as his old man.
Obviously as a responsible parent, it's my job, and my job alone, to dictate what I deem appropriate and what he should not be exposed to.
The main issue for me is context, there is a difference between violence as defence and violence as outright aggression. So while I'm happy enough for him to play through Mercenaries 2, I do get a bit uncomfortable when he plays Red Faction: Guerilla. In Mercs the lines of right and wrong are quite clear, but in RF:G he has a tendency to attack civilians who don't retaliate, and then keep shooting at them when they drop. This made me uncomfortable, so RF:G has been removed from his, already impressive, library.
To put it in some context, my son, Thomas, is the least aggressive 5 year old you could meet, indeed, he's soft as shite. But he does enjoy, nay love, the fantasy that gaming, movies. books, comics and his own imagination afford him, but by attacking civilians who don't defend themselves, or in the case of Mercs 2 get the hell out of his way, I felt he was getting a negative reinforcement of how to treat others, which goes against everything I have taught him as a parent.
A more immediate concern, however, is bad language in games. Anything with excessive or strong swearing is an automatic no-no. I also make sure that his gaming opportunities are varied and in some way educational: Scribblenauts, Elite Beat Agents (though he prefers Osu! Tatakae! Ouendan,) Little Big Planet, and Ratchet & Clank all offer different experiences that promote lateral thinking, quick reflexes or allow him to explore his creativity in other ways.
He has played through the opening of God of War (before the sex part) and is desperate to play Aliens Vs. Predator, but while the violence in the opening of GoW was all aimed at fantasy, Harryhausen style monsters the rest of the game gets far more visceral. And AVs.P is just a flat No Chance.
As he gets older, I won't deny him the thrill of playing games or seeing films he shouldn't, but I'd also have to be sure that he is emotionally ready to experience these fantastic works of art and continues to be the happy, social and considerate individual he is. And that, as with all of his upbringing, is my responsibility and no one else's.