I think it all depends on the level of maturity that you child has. You are the parent know that level. For us, our 9 year old is very mature for his age and plays Call of Duty Modern Warfare 2. In fact, he and I play together. It's hard to image not allowing him to play something like that as he knows the difference between real life and video gaming. Because we don't restrict it, we allow him to make his own decisions on how long he plays these games. I can tell you, it only about 30 mins every other day or so and sometimes not at all for days on end. I know some parents that suppress their boys so far...that nothing is reality. Boys will be boys and I'm all for allowing that. Some of the parents we know won't allow their boys have ANY type of fun with plastic guns, etc. The reality of it is that boys have it built into their genes to wanna shoot something, even if they have to use only their fingers as a gun. If those parents want to do that, it's up to them and they know their kids the best. I feel that it's all about being engaged and connected with your kids and not to shelter them from reality.
As a caveat, my boys play Madden 2011 amongst many other sports related games too. My 9 year old really doesn't have much time to play games anyhow, as he is very actively into playing 3 sports year round: Tackle Football, Basketball and Baseball. Plus if you throw homework in there (he's a straight A student and studies very hard), then he barely has time for video games.
I cap off warfare video game play with real world gun shooting and gun safety for my boys (and daughter when she is old enough). We shoot our guns regularly and teaching them what exactly a gun can do, how to handle one safely and how to handle one one for sport is all part of it for us.