You might as prepare your child for how the world is, it's corrupt and there is always a war going on. Not only does it prepare them for the world but it also helps with hand and eye coordination, eyesight, keeping bad kids off the streets, and also aiding in making our brains more flexible and coming up with solutions on the spot.
I have no problem with teens and pre teens playing cod as long as their parents think they are mature enough to handle it. I am not ok with kids that are still in elementary school playing it and communicating with other people on it. Younger kids who have not matured yet can't handle losing and dealing with things they do not like. And at the young age the game could change them negatively. However when kids are in there teens and there parents think they are mature enough to handle what other people have to say and the mature content in the game then it is fine for them to play it.
Look at any CoD box. The rating clearly shows either a '16', an '18', or an 'M'.
Why do you think they put that there? To make the box look nice?
Of course not, it's there to tell you that what's in the game is NOT APPROPRIATE FOR CHILDREN.
If that's the way it is, that's the way it is. You can't have sex before you're 16; but hey, my child is mature enough, so why not?
If kids want to shoot stuff, get 'em a toy pistol and play some cowboy games. Introducing themes such as terrorism shouldn't be given to kids >12, especially if they're the future of society.