I registered pretty much just so I can reply to this topic.
Here's a little background on myself to give everyone some perspective. I am 28, a father to a two year old daughter, and I've been gaming since I was 9. And yes, I've also played all the "violent" stuff when I was a kid too.
The big question that needs to be addressed first is, does media effect behavior? Specifically, does violent media (or rather, any media that showcases values contrary to what you want to espouse) cause violence or at least has an effect on how one perceives violence?
I think the jury is still out on that one on the exact nature of it's effect. But to say that it has NO effect would be ridiculous. And to say it absolute say over you is just outright ridiculous.
Of course it has effect on us. If a movie is capable of moving me to tears, what's to say that a game can't move me to anger over time? That is my own rationale. And because of this, I personally do believe that violent mediums can have adverse effects on kids. (Keep note, I'm crossing the medium here, not just talking about games)
But then the next question is how much? Can it be mitigated? Is it long lasting? Here's the reality for me. I LOVE gaming. And most of the games I have, tend towards having SOME kind of violent element in some way. As such, I will not pretend I am impartial to this. I think that past a certain point, within a certain frame of control, with the right amount of moderation, this kind of thing would not be an issue. But that's because I have those controls in place. I have the ability to mitigate the ill effects that I think this could have on me. My two year old daughter? She'll imitate behaviors off of her Elmo video right after she sees it (which is why I love the Elmo videos about eating vegetables, but that's a whole different thing all together), so naturally I'm not too keen on letting watch me play Tekken with King where I contort my opponent into human pretzels. Now, if she DID, okay, it happened. No big deal. I can still teach her that this is not acceptable behavior. (Though that would be incredibly hypocritical of me, since she just SAW me play a game that does just that) The fact of the matter is, before I feel comfortable letting her play games like that, I need to make sure she has the emotional and intellectual framework that allows her to understand what exactly these things mean.
The studies have shown one thing in common: video game is a risk, it is a factor. It might not be permanent, it might not be so far impacting. But let's face it... why would I want to even want to allow that to persist even for a short while? Why the fuck would I be okay for my daughter to think violence towards another human being is acceptable even for a short while? Why would I WANT to have to do damage control? That is my primary reason for why I won't let my daughter play violent videogames. At least, not until I KNOW that she understands how to moderate the feelings and all that junk on her own. Want to play WoW? Fine, show me that you can exercise enough self-control so you won't let your grades drop, and your other responsibilities slip. Want to play Tekken? Fine, show me that you understand that this is not what most martial artists do, or that this is not a realistic portrayal of what happens to people when they get into a fist fight.
Think of it like this. You wouldn't let your teenage kid drive your car until you know said kid can drive your car responsibly and not go total the damn thing, right? You wouldn't want your kid having sex until he/she has enough emotional/safety knowledge towards relationship and sex, right? If all of these things are contingent upon knowledge, why shouldn't videogames?
Am I trying to pass the buck for social ills on video games? (or by extension, media in general) Fuck no. The responsibility still obviously lies with the parents. We all know this. But guys, this is exactly the kind of strawman that HALF of the posts in this thread is fighting about. Yes, we get idiots out there who are negligent parents and would do anything to pass the responsibility for being a bad parent off onto someone else. But I'd wager that most parents out there understand this far better than you'd think. If my daughter starts playing violent videogames at say, 5, and decides in kingdergarden the best way to get what she wants is to beat the living tar out of her classmates, that's COMPLETELY my own fault.
And I am certainly not going to say that video games need to be censored. Categorized? perhaps. Let's be real here. I like my own games too much to ever want to see someone pass a draconian censorship law that would make all of my entertainment blande and dull. But I'd be crazy to not keep a close eye on what my daughter grows up with.