I've played the game, and I found it to be one of the most fun games ever. Yes, there is a lot of things involving drugs, drinking, language, violence, and a little bit of sex, but not much sex.
The Drugs: The main character doesn't once light up a joint throughout the game, and he hates drugs. A lot of people around him use it because of the high money it gets, but they do have a high price to pay in the end.
The Drinking: In the game, Niko can go to bars and drink with his friends, then become drunk. Lots of other characters get the ideas too, but it's not a major issue in it.
The Language: It's something you're gonna have to ignore. Yes, there is a lot of language, but Niko attempts to slow down on what he says to people. Lots of other people swear like crazy, one for example is Patrick McReary. He drops the F-word in almost every sentence he says, more than once.
The Violence: Another thing you'll have to get used to, but again, even Niko himself doesn't want to do it. In the game, you cannot hit any women. The main thing is, there are lots of characters that are thrown off buildings and some are even set on fire. One had both of those.
The Sex: This is something I would say you'd have to least worry about. You can have sex with your girlfriends in the game, but you don't see it at all, but you can hear them. Niko goes to strip clubs too.
Now for the good stuff.
The Good Role Model: Niko is a man who loves his cousin, but he hates how his cousin had lied about being rich, but promised they could achieve it. Niko is a Russian man at first looking for revenge on someone who killed twelve of his army friends for a thousand dollars, but has the choice of whether to kill them or not. If he doesn't, he actually feels very relieved. He doesn't like the drugs or too much violence, he tries to clean it up by doing violence to stop the evil people from smuggling drugs in the end.
The Game-play: It's pretty easy to learn if you've not played any of the GTA games. The first thing you'll notice though that the controls and camera have changed since San Andreas.
Bottom-line: I'd recommend this game for fifteen and older, but sure to teach them not to do the wrong things that others do in it.