Who are we kidding? All kids play COD. I started playing this when I was like 13. But, according to this website, it would definitely earn the NO KIDS rating because of the Violence and Language.
LANGUAGE: Tons of F-words and S-words. In the campaign, online, and in Zombies (new maps.) There's no getting around the swearing (although I think there is a graphic content filter option, but I've never tried it.)
VIOLENCE: Gratuitous amounts of gore. In the campaign you can shoot off limbs or heads. Grenades blow soldiers apart and rip stomachs out of bodies. On XBOX Live, heads can't come off, but explosives blow soldiers up and limbs fly (You even get awards for taking limbs off using the tank). In Zombies mode, heads blow up, stumps spurt blood, bottomless torsos crawl towards you, leaving bloody trails behind them. It's not any MORE violent than the story mode, but it's just that you're fighting SO MANY zombies that it seems gorier. Plus, it's more horror/fear - themed, so all around it just seems very intense.
The game overall really IS NOT for kids ... but hey, they're gonna be playing it anyway. Just be aware that it is incredibly graphic and there's tons of profanity.