Im a father of two and my eldest son who is 7, has been gaming for 3 years. I can't tell him about the realism of a game, because he's quite bright and is quick himself to point out the flaws in games that try to be too real. "OMG dad his lips arnt even moving at the same time as the words" Now on the subject of violence, well..His favourite movie is Blade (the first one) His favourite games are MW1 & MW2 (single player modes only), and the games he's completed unaided by me are vast and violent. The medium doesn't matter to me, what matters to me is that he has the sense (and he does) to know that what he's playing and what he's watching is not real.
Kids that shoot up places because they've played a video game, listened to song lyrics, watched a gory movie - there's something wrong with these people, it's little to do with the outside stimuli and alot to do with their defective brain and a warped sense of reality.
So yes, my clever and grounded child is allowed to watch violent movies and play 18 rated games.