I think the most important thing to get out of the way when discussing what makes a book YA is the fact that although a lot of people mistakenly think of YA as a genre it is actually an age group. So the simple answer to what makes a book YA is that teenagers and young adults are its target audience. But, aside from looking at what shelf the book is stacked on in your local bookshop, how can you really tell what the target audience for a book is?
YA books have teenage protagonists, but then so do a lot of books for adults. Some people would probably say that the language of a YA book would be more simplistic than in a lot of adult books. I think those people probably haven't read a lot of YA. They might also suggest that the subject matter isn't often as heavy, which proves that they definitely haven't read a lot of YA.
The problem with pinning down exactly how to tell if a book has been written for teenagers probably stems from the fact that it is a relatively new target audience, because teenagers are a relatively new concept. Your great grandparents didn't have the chance to be teenagers, until the middle of the last century most young people went straight from being children to being adults, without those in between years. They still had the in between years, of course, but they were never given a name, and so there was no need for a shelf in the bookshop specifically for stories about them. Now those in between years are acknowledged, they have a name, and books that deal specifically with the problems that you might encounter in that time between childhood and adulthood.
So is that what makes a book YA? That it deals with the problems that teenagers and young adults often face? I don't know about you but I was certainly never chosen as the face of any kind of rebellion during my teenage years. Although, of course, even the protagonists in dystopian fiction still have families and love interests to deal with. A teenager is a teenager no matter what circumstances they find themselves in.
Honestly, I'm not sure what does make a book YA. I would say that having a teenage characters who experiences typical teenage problems is good enough but I've read books with teenage protagonists that I wouldn't categorise as having been written for teenagers. So I want to know what you think. Do you think it's as simple as the story just being about teenagers, or do you think there's more to it than that?