20 is pretty high for a limit to the YA reading age, but I don't think this has anything to do with Faulkner or Dostoevsky. Lots of people, including adults, read YA as well as other types of fiction. If we think of YA as being popular just because it's easy to read I think we'll be missing the point. I'd say it's popular because it focuses on a really dramatic time of life. Becoming an adult, sorting out romantic relationships and independence and how to become an active participant in your own life rather than a passive one--that's some fertile ground for fiction.
That sounds like a guy I once worked with who stated that, when he was running his own company, he would make the series 24 compulsory inspirational watching for his employees...Jack Bauer as model employee!
