I gave a workshop on writing for teens this week. I still argue that you can break the stage beyond fluent readers into two parts:
Teens are still struggling with puberty, they are leaving childhood behind and they still retain an idealised view of adulthood. They like to see children having the power of adults. They will often read up about people a couple of years older than them.
Young Adults are post-puberty but still struggling with hormones. They are sexually enabled but may or may not be sexually active. They still have problems because their brain is still growing. They can suffer mood-swings, and tend to judge with their emotions. They often lack sleep. They are risk takers, because of the excess dopamine in their brain.
In their reading they seek books where the protagonists and other significant characters look like them. They need fast pace and emotional closeness, and this often leaves the writer at odds; these require two different writing styles. Often the author can combine both by making the stakes extremely high.
It is often difficult to place a YA novel in a genre; it is frequently multi-genre and multi-themed. The unifying factor is that it is a Biludungsroman, usually taking one adolescent theme and more often than not it is identity.
Also, this reader likes to have a substantial amount of control over the text.
The group on Saturday did get quite deep into this. It was most interesting.