The term "YA" obviously gets fuzzy at the edges. But to be clear: YA is a marketing term, perhaps more now than it ever has been. It's used by booksellers and librarians as a crude way to offer a space between things for children and things for adults. It can be a pretty arbitrary division. By it's basic criteria, The Adventures of Huckleberry Finn and To Kill A Mockingbird could have been called "YA" novels. But I don't think anyone would argue that those two are in a different league from, say, The Hunger Games or Twilight, both novels that have inspired a slew of copycat teen romance novels that have sort of hijacked the label.
For the moment, I'm going to side-step the tricky and baggage-laden term "YA," and just say that I'm interested in things that are either written for young people, or that hold appeal for them. That includes The Hobbit (review forthcoming!), which includes no characters between the ages of 12-18, but which is beloved by many pre-teen children as well as many adults; it includes Watership Down, which hasn't any human characters at all, much less adolescent ones; it includes The Lord of the Flies, which hits teenage protagonists dead on but which was written for adults, and it includes Never Let Me Go, a novel with characters that start as children and end as adults, but which beautifully details a heartbreaking coming-of-age story, and which is as subtle and captivating (and critically acclaimed) as anything else written for anyone. There are obviously a lot of bases to cover. And this doesn't even touch the classic canonical works that teenagers are required to read in school. If a young adult reads them, and identifies with them, can an argument be made that they are "young adult" books?
Having said all this, I've still got to reiterate that I'm no expert on literature written for young people; I've only read a lot of it. This isn't an essay, it's a conversation starter. There are plenty of other essays out there, and a lot of interesting research. I'm just interested to hear y'all's ruminations on how things written for young people can or should be different than stuff written for adults. Why do some people never graduate from young adult literature? Why do others refuse to touch the stuff? And why (if you agree with me) are both cases so unfortunate?