My SIL allows her three boys to play, and did so from about 6 years old, I'm a gamer myself and told her of the content. GTA is another one they play a lot. There a plenty of gameplay videos of Cod and GTA online that even if a parent isn't very good at games, it's easy enough to find what the content looks like.
Dd has most of the class on her Xbox live friends and out of 25 of them only 4 have been seen playing Cod, and two of those accounts are also used by Dad, but dd will say that you'd think everyone played the way they went on.
There is a plants vs zombies game called garden warfare, and dd plays that, it's very similar in play style to cod but without without the violence and gore and inappropriate language. It has a 7 age rating.
I used to play cod a while back but there where a lot of children playing and they used the worst language I've ever heard. Def worse than any adult I've played.
Someone said earlier that Xbox live is dodgy, it can be dodgy if you don't use it properly, it's no more dodgy than being online in general, if you talk to your child about not accepting random friends and not talking to people they don't know, put controls in place etc then it's fine.