Originally Posted by Warren_Paul
The theme mainly, if it concentrates on stories with challenges that are relevant to youths. Also the protagonist has to be around the age of the target market you write the book for.
Writing style also plays a part, but it's not what decides YA or not, just needs to be deeper for adult fiction than it does for YA.
Deeper? Deeper how? Psychologically, religiously both? Or basically just deeper than whether or not the main character gets the girl so-to-speak?