I might be quoting the unpopular opinion here, but I tend to disagree when people call YA & adult books exactly the same. I honestly don't think they've read enough adult books.
Maybe the majority of (older) teens can handle everything adults can, but the genre can't, at least for now. There's simply a lot more gratuitous/explicit sex and violence in adult books -- enough that there are entire genres called erotica and horror. Sure, YA horror is fine, but scroll down a few threads -- we just had trouble coming up with more than a few contemporary YA horror novels. And sex in YA is also fine, but never gratuitous sex; and even when the camera doesn't turn away (Little Brother, Anatomy of a Boyfriend, etc.) it's still not porny or even graphic.
If you disagree with me, go get
The White Hotel by D. M. Thomas and
Blood Meridian by Cormac McCarthy (both literary, not horror or porn). Then, maybe after the nightmares have stopped, try to tell me that sex/violence in YA parallels adult novels.