Someone asked on the comments section a couple of days ago: what's the criteria for a work to be classed as "young adult"?
I know pretty much zilch about the subject, but I suspect there is no precise criteria. Rather, an editor - or an author - will make a decision, and if the marketing people agree, then that's the way it will go. I suspect that there are times when everyone is caught out by what happens next. I mean, they had to put out "an adult edition" of Harry Potter! Huh? It had a different cover - so adults wouldn't be embarrassed to be seen reading it?
Trudi Canavan's "The Black Magician" trilogy was not really marketed as a YA in the beginning, but once the book hit High Street in UK, it was obvious that it was having huge success among the slightly younger reader. A YA edition was brought out - exactly the same text, but with a different cover, and pushed to bookshops as YA, which would probably place it in a different section of the store.
So what is it about a story that makes it YA, not adult?
Perhaps YA authors can comment! But here are some of my guesses:
1. Language - not too much of the more vivid curses
2. Age of main protagonist(s) - not quite adult
3. Plot not X rated - if there is any graphic sex, it will be more of the "this is the first time for me too" variety.
4. Probably deals with at least a few "coming of age" issues, if only in passing. Such issues are often the main concern of the book.
And what's the difference between children's literature and YA?
1. Age of main protagonist(s) is younger
2. Language even more circumspect
3. No sex and probably not too much kissing either!
4. Issues will be the kind of thing that is of interest to the targeted age group.
Of course, when I was a highschool teacher, way back when, the kids could run rings around me in their knowledge of graphic swear words, not to mention slang for every conceivable part of the anatomy and the variable uses the said anatomy could be put to, and what you called it when you did...
But even so, YA books are unlikely to be raunchy. Young Adults have to be protected from that kind of thing, after all.