Video games are VERY violent and I think that only mature audiences should be able to experience them. How mature that is however, is up the parent/guardian of a child. My parents let me play doom while I was very young but it was because I've never showed any violent tendencies. I've never been in a fight (except playin soccer, no fist fights but I've hurt plenty). Some children however, do not have the full capability to determine between reality and virtual reality. It appears nowadays that parents are a lot more carefree of what they let their children watch, eat, listen to, and participate in and it kills me to see all the violence in the world today that is marketed towards the youth. It's not just video games, it's everything around us. I believe in freedom of expression, but I also believe that children should NOT experience all the violence, sex, and drugs on television, music, and in video games. I look at it as the adults of today to make the right decision towards what their children should experience at what ages.
I could get into the different levels of violence in games from Mario Bros to Doom 3. I won't though because I'll just start rambling even more.