Let me begin by saying this - Fallout 3 is COMPLETELY different than its predecessors, Fallouts 1 and 2. In the first two fallouts, the world is alot grittier, alot more depressing, alot more violent, sexual and disturbing than its Bethesda-made rebirth. That's why I believe this game is terrible - except for its massive amounts of violence, it cannot live up to the awesome details of its forefathers.
Now, onto the game itself.
Sex, drugs, violence... What's there not to like? But besides all of these malefactors (all of which I hear about, see or use at school on a daily basis [I'm not a druggie, though]), there are some valuable Karma-based lessons to reap from this post-apocalyptic pie. It's really quite simple - you do bad things, bad things happen. Do good things, good things happen. So really, because of these valuable lessons and the use of factors common in teenage society, I believe anybody over the age of 15+ can enjoy it.
Just please. For the love of God, these players need to play the first two Fallouts. Sure, you may need to have a higher tolerance, but the classics are far more immersive and remain true to Interplay's original canon.