The people who define adolescent literature are publishers, librarians, booksellers, teachers, and adolescents, in that order. Adolescent literature can be divided into two categories: “books written specifically for adolescents” (the traditional books that get marketed towards teenagers), “books written for adults that have adolescent protagonists” (I’m thinking specifically about Stranger Things which was originally marketed towards adults but also has middle grade-aged protagonists and teenage protagonists and as a result those age groups have also watched and enjoyed Stranger Things, but books with teenage protagonists aren’t always for teenagers; Purple Hibiscus by Chimamanda Ngozi Adichie is about a fifteen year old girl, but the book is marketed and mostly read by adults), which can also often be broadly categorized as “books written and marketed for adults but which adolescents have made their own”; specifically Catcher in the Rye, which was written before “adolescent literature” was even a marketable category of books!
The word “adolescent” implies “teenager or middle-schooler” to me – someone between the ages of eleven and nineteen. Can middle-grade novels be categorized as adolescent literature? I think that “adolescent literature” applies to both “young adult” (or YA) novels and “middle grade” novels. And obviously, if teenagers can read books for adults and make them their own, adults can read books for teenagers and make them their own as well.
I think the most notable trope in adolescent literature is coming of age, of course! The transition between one stage of your life (childhood) to another (adulthood). I think this results in a lot of crossover reading between multiple ages, especially with queer adults reading queer YA books, because they weren’t necessarily able to come of age as themselves when they were adolescents themselves. Other fairly famous and popular tropes include: love triangles (Hunger Games! Twilight! A series called Matched I was really into as a teenager!) The Chosen One or fulfilling your ‘destiny’ is really popular as well, I think especially because it mirrors and matches The Hero’s Journey and the “collective unconscious”. Also, obviously, dystopias were really big for a minute there, and maybe still are? Divergent, Hunger Games again, the Maze Runner books, that Matched series I really liked, Uglies/Pretties/Specials; all books about revolutions as in the turning (revolving) of a world into something else.