1. Young Adult is not a genre. We hear that often — “the YA genre.” You’re wrong. Don’t call it that. Stop it. But seriously, Young Adult is a proposed age range for those who wish to read a particular book. It is a demographic rather than an agglomeration of people who like to read stories
2. The average Young Adult novel probably hovers around the 70,000 word mark — shorter if it leans away from genre and toward literary. Particularly, for the first book in a series.
3. They also tend to be more quickly paced and with a great deal of dialogue. Some young adult books read with almost the spare elegance of a really sharp, elegant screenplay.
4. Adults are rarely the main characters of a young adult book. Why would they be? They don’t have teen problems. They’re witnesses, at best. That said, adults can be the supporting characters (though usually still peripheral to the teen world — teachers, parents, older siblings) and they can also certainly be the villains.
5. Adults read a lot of young adult fiction, particularly “cross-over” fiction that leans toward the higher end of that teen age range. One might speculate adults like it because it recaptures some part of their youth. Or that adults are frequently not as grown up as they’d prefer these days.