One of my coworkers, who is also a writer, and I were discussing this the other day. There are some people, publishers, and groups who consider Young Adult (YA) to be a genre. But Maria and I were talking about how that's really kind of silly; sure, YA is usually about a teenager and topics that teens are interested in, but aside from that, there are as many genres as there are in adult or children's literature.
Really, when you get down to it, YA is a rating. It's a marketing tool. It's a lot of things, but it's not a genre.
Before we get too far into discussing what YA isn't, maybe we should define what YA is. And the problem is, there is not a universal definition. "Young Adult" is defined as literature written for, about, and sometimes even by teenagers. The main character falls into a certain age range (which I'll expound on in a moment), and deals with things of interest to teenagers, from finding yourself and coming of age, to more serious topics like bullying and first sexual encounters (which can include negative sexual experiences like assault and its aftermath). Sometimes these books are edgy, intentionally pushing the envelope, and sometimes they're just girls or boys of a certain age chilling.
You would think the term "teenager" is pretty straight-forward, but for terms of defining what qualifies as YA, it's not. I used to work at a public library that defined YA as appropriate for 11-17-year-olds. However, many publishers define YA as being appropriate for 13-18-year-olds. There is a big maturity gap between 11 and 13, not to mention a legal gap between 17 and 18. That's on top of the problem that even going with 13-17, that's still a big range of what may be appropriate or of interest.
And that's before you even touch on the YA holy grail - the series. Take Harry Potter for example - in the first book, Harry is 11; in the last he is 17. By the definition of the library system where I used to work, that is YA. And yet, the first 3 books are classified as "juvenile" (5th grade and under), while the last 4 are in YA. Partly, this is because the first few books are relatively light and things start to get darker, serious, and more mature at the end of Prisoner of Azkaban. Similarly, the Rangers Apprentice series is split between J and YA, though if I recall, the main character is actually a few years older than Harry at the start of the series. But then you have other (admittedly older) series that are all housed in 1 section or the other regardless of the broader age range. The Little House books are held entirely in the juvenile section, despite the fact that Laura is 5 in the first book and the last book covers the early years of her marriage, from the age of 16 to 20. Anne of Green Gables and its sequels, by comparison, are all housed in YA; Anne is 11 in the first book and her books also follow her into her marriage. Really, if you want to get nit-picky about topics, the last couple Little House and Anne books should be classified as "New Adult."
"Well, now, wait, what's this 'New Adult' thing?" you might ask. New Adult is a growing classification. New Adult, from what little I've read, is geared at 18-25-year-olds, and deals with things like leaving home for college or a first job, first serious romantic relationship or the early years of a marriage, sometimes even dealing with the birth of a first child. Because of the material and the age, this classification can also usually get away with more graphic sexuality than books aimed at high schoolers. (Not that there's intense sexuality or overly mature topics in either Little House or Anne, but 4th graders may have a hard time relating to Laura struggling with making jelly on her own for the first time, whereas older readers relate to the fact that they are having to cook, clean, and "keep house" without Mom to help out.)
A big part of why I bring this topic up is that the 3 novels I'm working on are right on the edge of YA, teetering into adult or NA. By straight age my novels could be classified as YA or NA - for some reason, I'm writing about 17- and 18-year-olds. I'm also writing in a "period" where the characters are considered adults with responsibilities as young as 16 and 17. But then there's also the fact that even though there are going to be some mature topics, these books aren't going to be edgy or overly graphic.
I have an acquaintance who writes, among other things, children's novels. She says that the target audience of YA should be 1-2 years younger than the main character. (Which I could argue with, given that there are an awful lot of adult readers who read YA, but from a marketing standpoint, I can see that.) But by that argument, every book about an 18-year-old should be read by 16-year-olds; every book about an 11-year-old should be read by 9-year-olds, and I don't think that's always a straight correlation.
So, really, when I go to publish these books, I'm going to have a hard time if someone asks me flat out "are you marketing this as YA?" But I guess I need to actually finish writing them before I worry about that.