I know I'm not a parent or anything, but I do know that I love these games and I know I would never let my kids play these games until they got to a certain age. The fact of the matter is, you're really introducing your kid to hostility and very adult language and topics without any care or shield. 8 Year Olds should NOT be on Mature Games, they should be playing what I grew up playing and that would be Mario-something, or even Go Fish. I understand if they're annoying, (trust me, I know all about that, I'm the one stuck with them while you're sitting in the other room sipping your frappaccinos), but don't dish them out to other people.
Back to what the studies said about kids playing mature video games end up "more violent".. I actually agree. I know a ton of kids on my block who grew up playing video games and I can't count how many verbal and physical fights they get in at school, their grades suck, they have a low attention span, and they're tempermental... can't even talk to them without them having them rant like Mel Gibson on steroids.