I don't subscribe to the "kids are stupid and think video games are real" theory. But I do think some games, like movies, are rated mature for a reason. Regardless of how "mature" a parent believes an 8 year old is, I doubt they would rent them a movie with sex and extreme violence even if dad was watching him.
So it bothers me when they come in and buy Grand Theft Auto vice city for their kids. When I warn them about the mature rating they are almost offended.
(Mom is buying guide for 8 year old)
Me to kid: "Is this your dads game?"
Mom: "No, it's his, but we watch him and he knows what he isn't allowed to do. Like NO GIRLS"
Kid: "YEAH! I KILL ALL THE GIRLS I SEE!!!!"
Um yeah. That's a lot better.. x_x