I was playing Left 4 Dead late at night a while ago.  Trying to 'Expert' the 'Dead Air' campaign.  Met a few complete idiots on the road to doing it, and some pretty cool people too... Eventually though I was left at the finale with two bots and this really cool girl... Or at least I thought it was, due to the pitch of the voice.   After some general conversation 'she' turned out to be a 'he' some kid in Canada of all places.
 
Now he/she is somewhat irrelevant, it just illustrates the point about how young this guy was to be playing a cert 18 game.  I quizzed him on his thoughts on this, (He turned out to be 12.) which was that, yes, Left 4 Dead is also cert 18 in Canada, but, and I quote, "Left 4 Dead is awsomeness so I just play it anyway." 
 
Well, erm, okay... Having spoken to guy, and I still see him online from time to time, I'm not convinced he's been severely psychologically damaged by playing it.  Whether because of the violence in the media these days kids are desensitized to it much earlier than they once were... Or maybe some kids mature at different ages, or maybe he will grow up to be a psychopath?  I don't know...
 
I do know, despite quite happily playing video games with my 10 year old nephew , I would not be comfortable playing Left 4 Dead on split screen with him... Okay 10/12 - same, but not the same I know... I wouldn't be happy with my own little girl playing Left 4 Dead if she was 12 (She's 3 at the moment.)...
 
I suppose, the graphics on the whole aren't actually all that scary, certainly not that realistic.  If anything it's all a bit cartoony.  A 3 year old would find it scary - but I'm not convinced it is realistic enough for a violence desensitized 12 year old to be scared of.   So should they play?  On the one hand I think no - if you think about what Left 4 Dead is representing, then it's quite horrific, a zombie infestation causing innocent people to turn psycho - and the survivors have to murder innocent people to get to the safe house?  That's not nice... 
 
The question is, does the average 12 year old think about this?  I'm guessing not - I'm thinking in the spirit of the best George Romero Classic they are just playing the game, point and shoot at the target - and enjoying the story, the plight of the protagonists and wanting them to survive... Or just being silly - going on expert and killing all the other players just to be a dick....
 
Does that mean we should scrap age certification for games like this?  I mean how much harm does showing young children depictions of violence actually cause?
 
Personally I think age certification should stay and be enforced.  There's enough violence and conflict in the world for us to want to protect our children from it for as long possible.  It might not damage them, but it's still nice to think they can spend their life as a child in the naive state of not truly understanding or being exposed to violence, because once they are adults there is no getting away from it.