First off, video games are hard because of online play which can contain profanity and arguments from other users. I'm going to speak about the actual, non-online gameplay. It really comes down to how far along your child is. Some teens and even kids can handle warfare and violence and it is fine for them as long as there is no nudity or profanity. Out of all things that make something rated mature, violence is probably the best for a child to be exposed to - still not good, but better than the other two.
Thankfully there is no nudity, and the intimate scenes are not too bad. That in itself is probably pg-13, but the violence and profanity is R rated, no doubt.
Although there is a lot of killing, 95% is done against those who represent an oppressive tyranny caught in a revolution of sorts. The game is also a great history lesson, and although some things are not acceptable today, they were back then, and the game simply is trying to be realistic.
All in all, it comes down to your own discretion.
What other families should know
Too much violence
Too much sex
Too much swearing