Nowadays, "young-adult fiction" includes every type of story, including post-apocalyptic and fantasy, says Forest Of Hands And Teeth author Carrie Ryan. And then readers reach adulthood and we suddenly expect them to choose their genres.
One of the great things about YA is that all the books are shelved together (or split into "realistic books" and "SFF books"). There's no distinct category for high fantasy or science fiction or romance or historical or contemporary issues. YA books can pull from anything they want so you can have a book that's a post-zombie apocalypse romance that may or may not have a happily ever after or you can have a book about a school where cheerleaders are high tech spies thwarting an assassination or a book about killer unicorns and the virgin descendants of Alexander the Great who hunt them or one about a high school valedictorian who ends up pregnant and doesn't know what to do.
Sometimes I wonder when it happens that we "graduate" from YA and "choose" our section of the bookstore. Sure some people read across all genres, but a lot find their niche and stick with it. I was in high school when I pretty much started choosing books only from the Romance section and now I realize just how much I was missing by not branching out. On one hand, it's nice to walk into a bookstore and straight to the section that has books you pretty much know you'll love. On the other hand, I kind of like it when all our books are jumbled together and as a reader you don't know what you're going to get other than (hopefully) a pretty awesome story.
To me, this is one of the greatest thrills writing (and reading) YA. I don't have to worry where my book is going to be shelved or how the booksellers will be pitching it (my next release has the tagline "Eat, Prey, Love"). I get a huge thrill when readers tell me that they never considered they might like zombies or horror or post-apocalyptic romance but that they enjoyed my book and that it made them look for others like it.