The divides are artificial, the boundaries are blurred, it's pretty much a mess.
At work, there are several divisions of kid books.
- 5-7
- 8-12
- Teen
- Young Adult (for the past several years this was called "books with byte", because it was DOMINATED by Twilight style work. At the end of the day it's pretty much the same as teen, except YA has a definite gender bias, more of which later.)
Now, Darren Shan is put in teen. His first books were 50k word count (very much 8-12 range), the language wasn't advanced, the themes weren't particularly YA focused. They're quite similar to Joseph Delaney, who is classified as 8-12 readership (the same as Beast Quest, books that're 10k word count). There's no real reason for Shan to be in teen and Delaney not.
Harry Potter is classed as 8-12, and for the first 2 books that may be right. But from the 3rd book on the word count becomes huge, the themes become far more adult, but it has to go somewhere, right?
Gaiman's Coraline is put on the shelves in the 8-12 section, or if they put a different cover on it it goes with adult fantasy.
"Books with byte" has had its time. The dystopian future is the next big trend, coming off the back of the success of Hunger Games. Whether it continues to do well depends on the success of the Divergent and Maze Runner movies. Otherwise there'll be a void to be filled.
If you go into any high street book store, the majority of YA books tend to be female focused. There's a massively disproportional amount of female agents, and I don't know if that has an effect or not. But at the end of the day they put out what sells and, at that age, it'd suggest females are reading a massive amount more than males.
Phillip Pullman's His Dark Materials is classed as teen, but it's a coming of age trilogy, the level of prose is above and beyond 99% of what is sat in YA.
So, ultimately, there's very little to clearly define what is YA. Word count doesn't have a bearing - 50k can happily sit anywhere except the 5-7 range. Quality of prose has no bearing, or Pullman would be in teen at the very least. Topics covered also seems to have little bearing.
Mark Lawrence's Prince Of Thorns isn't dealing with an average teen. By all accounts Jorg is as psychopathic as Andy McNab. But does that mean it has any less right to be called YA? I don't know. I've had 10 year olds bring it up to the counter, asking if it's suitable.
The only thing that YA books seem to have in common in the age of the protagonist.