Like hatesponge, I promised myself I wasn't going to become embroiled in this, if only because I don't have the next 3 days free to argue on MN.
However, for every piece of research you can find saying that these games are damaging, you can find another piece disputing it. Some research has indicated that children who are prone to aggression benefit from these games because it allows them to get it all out in a controlled environment rather than irl. I don't know how true that is, but it does illustrate that the research offers varying pov.
People are very dismissive of anecdotal evidence, but imo it has as much validity as the educated guesswork of the researchers. I know lots of children who play these games (admittedly a bit older than 8 ) and none of them are violent/antisocial/on the verge of torturing small animals, prior to commencing an adult life as a gun wielding maniac.
Children are individuals, which is why their parents are best placed to judge what is appropriate. Guidelines exist so parents have an idea of what they are purchasing and also in recognition of the fact that not all children develop at the same rate and some will not be 'ready' for these types of games.
I vehemently disagree with this notion that if you allow your child to do one thing that you automatically allow them to do everything. I am an educated, intelligent woman and a conscientious parent. I often take a different view to the people who decide classification for film/games. There are films which have quite a low classification that I would not be happy with my dc watching (latest Twilight film for example, which I think was 12A), so i consider myself better placed to decide than some anonymous board of people who don't know my family.