That's a reference to a post I wrote long, long ago called "Why YA?" At the time, I was working on what I thought was a YA novel. But the words Young Adult infer a range greater than the genre actually encompasses.
Notice I called it a genre instead of a category. This is based in part on the sense I've gotten from reading blogs, reviews, articles and other flotsam on the internet. It's also based on responses I got from an #askagent question on Twitter. (Okay, so I only got three responses from agents, but two of them said genre. One said age group. The non-agent responses were evenly split.)
The expectations for YA fiction have less to do with age group and more to do with the type of story you might expect to find in that corner of the bookstore. If it were strictly an age grouping, it would include a broad range of fully-fledged genres. Instead, it includes sub-genres that resemble their non-YA counterparts, but also have certain hallmarks qualifying them to be YA. What are those hallmarks?
High school is the first one that comes to mind. The characters have to be in high school or at least in some situation vaguely like high school. They must be of high school age.
The protagonist must be in a romantic relationship by the end. Love triangles are common.
The plot and the stakes, however grand or perilous, must not outshine the protagonist's personal life and dilemmas.
The protagonist is usually female. The author is usually female. The readers are usually female.
Probably some others.
There are, of course, exceptions for every one of these. Early on, I was misled by some books I found shelved with YA that seemed quite different. But for each of these, there are ten or twenty that fit the mold perfectly. Also, these books are mostly by well-known authors whose careers began before the rise of YA as a genre. Others were written for adults and changed to YA by the publishers, (who know that it's mostly adults reading YA anyway.)
If you want to write YA, it's not enough to simply write for an audience of young adults. You have to write for the YA audience, which does not include most young adults. (As I've said before, if I were sixteen, I wouldn't be caught dead in the YA section of a bookstore.) There is no age grouping for teenagers and young adults in general.
And maybe that's the way it should be. Good storytelling is good storytelling. There's no good way to divide books into age groups. Some of my favorites are considered middle grade, and I've been an adult for at least a couple years. They have broad appeal. They fit into a genre, (usually fantasy,) but not an age group. And how young is too young to read Ray Bradbury? Nothing he wrote was aimed at juveniles, yet that's usually when we fall in love with his writing. And when we're grown up, we can read it again and see things we missed before. And while I was reading—I mean critiquing—Krista's middle-grade manuscript, I entirely forgot that it was middle grade. It was just good. Anyone of any age could love it.
Books is books. And I don't actually write YA. I just write. Look for my books in the sci-fi/fantasy section. (In a few years.)
p.s. I'm not very widely-read in YA and certainly not an expert. The opinions expressed here are those of myself at the time of writing and not necessarily those of my past, current, or future self. Mileage may vary.