This is a topic that's been spinning in my head for awhile now: What is YA?
There's the obvious answer. YA is young adult literature, books intended for teens typically between the ages of 13-17 (give or take a few years). And there's the assumption that this includes the typical topics: teen love, "growing pains," and angst.
And that's as far as most people go when considering YA.
The problem? YA is a misnomer. YA literature is not books for young adults. It's not. YA books aren't book intended for a certain age (just look at the varying degrees of censorship applied to many YA books to know that).
No, YA is a genre, not an age range. And to have good YA does not mean that you have to follow a set of guidelines and write down to teens. Instead, it's about fitting the tropes of the genre, and those tropes have nothing to do with age.
Character-based Story: YA books focus on characters more than setting. This is especially true of fantasy and sf. Adult sff is much more concerned about world building--one reason for the door-stop sized thickness of the book. YA sff isn't worried about the legends behind the magic, the origins of magic, or the development of the world--unless it directly relates to the characters. YA sff is much much much more concerned with the inner workings of the character's minds, not the inner workings of the world s/he inhabits.
Fast Plot: I'm not saying adult books don't have fast plot. But--like adult thrillers, suspense, and some romance sub-genres--YA is more focused on a page-turning plot than a literary level of contemplative wording. A YA is--like those other adult genres I mentioned--likely to end a chapter with an explosion or cliffhanger much more than a subtle allusion or rhetorical philosophical question.
Relationships: This is, I suspect, where the stereotype of YA having to have a boy-girl love factor in it. That's not true. But YA does tend to focus on some type of relationship: either a love one, or a relationship between friends, or even (such as in Laurie Halse Anderson's SPEAK) a relationship with one's own self. YA lit is character based, and nearly always, that character based format is developed through some form of relationship. (Compare this, for example, to Tolkein's LORD OF THE RINGS--although often lauded by teens, this series is not YA in part because the relationships between the characters, despite the foundation of the story being in the Fellowship, is minimized in comparison to an almost exclusive focus on Frodo's internal struggle.)
Size (maybe) Matters: YA does tend to be shorter than some adult literature. But not always (see: TWILIGHT, HARRY POTTER). The point is not that the stories need to be shorter, it's that in YA, every word counts. If you have 500 pages of words that count (which TWILIGHT and HARRY POTTER certainly do), then include it. If you've only got 150 pages of words that count, then do that. You will not see fluff in much YA, because the bottom line isn't the size of the text, but the impact of the story.
Any Topic or Genre Will Do: YA isn't about fitting one role, story-wise. You can have YA romance (Sarah Dessen), historical (Markus Zusak, MT Anderson), fantasy (Kristin Cashore), science fiction (Scott Westerfield), futuristic dystopian with a mix of romance and thriller (Suzanne Collins). The point is: in YA, it doesn't matter what your subject is, as long as you've got the characters, plot, and relationship to tell a good story. And don't think you've got to limit your story to a certain "age-appropriate" plot. YA runs the gamut of "acceptable": suicide (13 REASONS WHY), rape (SPEAK), eating disorders (WINTERGIRLS), homosexuality (THE BERMUDEZ TRIANGLE), incest (V.C. Andrews), and children thrown into an arena to kill each other in a bloodbath (HUNGER GAMES).
As I hope you can see, YA is not about a certain age range. Instead, YA is more of a genre in and of itself. Like romance, historicals, and suspense, YA has certain tropes--which are not restricted by age. Unlike childrens and MG books, YA is not limited to certain topics, and (despite the crazy censors) there is no real measuring stick for what's appropriate and what's not. Instead, YA literature is much more about character-based fast plots with a focus on relationships and making every word in the story contribute to the overall story. It has nothing to do with the age of the reader, despite where it's shelved in the bookstore.