Over the years I've heard from people who made it clear that they defined Young Adult fiction as books that didn't include much sex or so-called mature subject matter. I heard of one writer whose publisher was going to market his genre book as a crossover to YA because it was "clean." These folks were living in a dreamworld, of course. In addition, they didn't know what YA was because they didn't understand that it's supposed to be about something, about something different from books that are for other readers.
I frequently find myself explaining that theme is important in YA, since theme, essentially, is what a book is about. A really good YA book includes classic YA themes, such as separating oneself from family, seeking a path in life. Usually the people I'm talking to don't read much YA, so they have no idea what I'm talking about. In fact, I know some of my family members think I'm making this stuff up as I go along.
Oddly enough, I'm clearest on my thinking about YA themes when I'm reading a M(iddle) A(ge) book, as I am right now. MA books deal with disappointment. They deal with coming to terms with what your life has been. That's a very strong contrast to YA books that deal with what your life is going to be.
Books directed to various stages of life address themes important to those stages.
Childhood: After having been the center of the universe in order to survive (cry-get fed, cry-get changed, cry-get attention),I find out that this behavior is no longer going to work for me. How will I get along with others at school, day care, Scouts, the world? How much am I willing to conform in order to get along with others?
Young Adult: Separation. How am I like/different from my family/peer group? What will I do with my life? What will become of me?
Twenty/Thirty Somethings: Life sucks. Shouldn't someone have told me? Now what?
Middle Age: Assessment. How have I spent my life? Did I do good? Is this what I wanted? Is it too late for me?
Older Age: I'm too old to give a damn. My last shot at happiness and fulfillment.
YA fiction isn't the only kind of literature that addresses concerns/themes of a specific age group. Every age has its themes. It's much easier to understand what YA literature is when you understand it in relation to these other types of literature.