every single teenage novel i've encountered has either a teenager or a fantasy figure who can pass as a teenager (halfling, hobbit, dwarf) in the main role.
and really, every single one of them. Harry Potter, Artemis Fowl, Anthony Horowitz, the Hunger Games, the neverending story,... I can't think of a single book aimed at teenagers or young adults that has an adult as the protagonist. even the Hobbit has, well, a hobbit as the main lead.