It depends what you're doing in the game.
My youngest son, 8 years old, plays "the simpsons hit and run" on ps2, a game rated 7+.
There he kicks all the people walking on the street or crashes them with a car.
Behind my back, he played "grand theft auto", rated 18+, where you can hurt people too, but there he loves driving around with an ambulance or bus. He dont hurt anyone.
Now, I let him play the 18+ game when I can keep an eye on him. Everybody happy.
hans