Ahh this old thread again. A normal M-rated shooter (nothing overly horrific/scary, mind you) should be perfectly fine for kids over the age of 8 to play as long as there is proper guidance from the parents about what is, and what isn't socially acceptable behavior, and the line between fantasy and reality. A kid is in no way going to be "emotionally damaged" by playing a video game unless they're incredibly sensitive, in which case it's the parent's responsibility to take that into account.
People just don't give kids enough credit anymore. Hell, if anything, not giving kids an outlet for aggression is probably far more harmful than letting them play a video game. It's not like they can play any other games anymore, like tag or dodgeball for the very same reasons. Wouldn't want the little tykes to get all emotionally scarred by the "exclusionary nature" of dodgeball, after all =P but I digress.
I feel bad for kids these days.