I think there is a massive element of common sense here..
With my kids, I don't necessarily look at the age rating, just my own sense of suitability, I do not let my 4/6 year olds play violent FPS games, I do think that it would slightly affect their development in a negative way, but I'm not sanctimonious with it.
On the flip side, my Brother lets all his kids play every latest FPS game going, irrespective of age rating, even when his youngest was 6, I never did agree with this, but just let it be, however he is the one that has a go at me because I don't let my kids play COD etc as I feel they are too young and impressionable.
As a laugh, when he got quite angry with me because I dare say "No, I don't let my kids play COD etc because I think kids at a young age are quite impressionable", he said that was a load of balls, so half jokingly asked his 8 year old son "If you got in a fight at school, would you whip your AK47 out and shoot them?", to which his son replied "No, of course not Dad".. So my brother then looks at me all smugly, however his son then added "But I'd get my rocket launcher out and fry his ass"..

I just LOL'd massively at that point...
His kids are fairly well grounded, so I don't honestly think they'll grow up to be monsters, but if you see the massive tantrums they get if he switches off their PS3/Xbox, with them wielding anything around them in some mentally angry manner, you can't help wonder if the violence they see/do in the games does fuel/train this type of response at times..
I'm on the fence, I'm not against others doing it unless it's damn obvious it's affecting the kid, but I'm certainly showing some common sense with my own kids..