Great question.
My son is 16. When he was 10, I didn't have a problem with him playing games with violence so long as it wasn't over the top, head splitting open type stuff.
Also, I wouldn't let him play that game if it had vulgar language, and COD games definitely have them during the story mode.
When he was 10, I remember him watching Rambo: First Blood with me. I personally don't think its that big of a deal. If you raise your kids right way and explain things to them, you don't have to worry about the effects that a little violent scene might have on them.
Now, a movie like The Departed or Saw at that age? Absolutely not. Playing a game like GTA as a child? No chance in hell.
I saw Die Hard with my dad when I was 10, and I never thought killing was cool nor did I want to immediately go out and get a gun. It all comes down to how your raise your kids, the examples they see on a daily basis, and the values you instill in them.
Playing a video game where soldiers get shot isn't going to change that. Like I said, the key thing for me was how that violence was presented and what other violent and/or vulgar aspects are reflected in that game/movie.