What makes a book a young-adult novel, versus say an adult novel about young people?

I've been thinking about this question a lot lately for several reasons. For one, one of my readers asked me if his book would qualify as YA because its narrator is 18 even though the narrator does some very un-18-year-old things. Another reason for this is my own books, which seem to constantly skirt the line between YA and adult literature even though I always set out to write YA. As I was writing If I...