I think that age ratings are utter rubbish. For this game, it's an 18 because of violence, drugs, swearing and sexual themes. The violence: It's nothing I haven't seen on other FPS games, and this only needs to be a problem if you are very easily scared and traumatized. The drugs: There's only smoking, which is something everyone sees on the streets every day. The swearing: I already know all the swear words, so why should this be a problem? If you're worried your child might use the words more frequently after hearing them a lot, just mute it. The sexual themes: No sex or anything like that, just girls dresses in mildly suggestive clothing, which you see on the streets every day.
There's also a graphic content reducer in the options menu, which bleeps out all of the swear words and blacks out the screen to block out the really graphic violence.
In short, I believe this game is fit for children 10 years old upwards, provided they are mature and not very affected by pixelated violence.
The game itself is utterly brilliant. Zombies has returned better than ever, with really good new maps. True , they're smaller than the black ops 1 maps, but they're way more intense and fun. The problem is, all the fire flickering makes it lag quite badly, worse on the PS3. The online gaming is awesome, especially with the revamped Nuketown map, Nuketown 2025. There is the possibility of online bullying, but you can just mute whichever playes you think are annoying you. This is the best CoD experience yet.