Françoise Sagan, Walter Farley & Mary Shelley

Still thinking about young authors: Maureen Daly was 17 in 1942 when she published Seventeenth Summer and invented teen literature. Unless you want to say that S. E. Hinton invented it at age 16 with the publication of The Outsiders in 1967.
       (We may be overstating, but it's easy to find good academic authority for the idea. As an article in the ALAN Review (Assembly for Literature on Adolescents) says: "Until the mid twentieth century, adolescents didn't really exist. There were two groups: adults and children. Some date the invention of teen literature to the publication of Maureen Daly's Seventeenth Summer in 1942; ... For others, S. E. Hinton defined the category with The Outsiders in 1967.")
       But young authors don't write only teen lit, nor only lately. Mary Shelley was 17 when she wrote Frankenstein. Françoise Sagan was 19 when she published Bonjour Tristesse. There's an article in a recent Christian Science Monitor about Helen Oyeyemi's new novel, The Icarus Girl. She's 18 (well, maybe a little older now, 18 when she wrote it).
       Salt Lake City Public Library has a list of young authors. People don't read Seventeenth Summer anymore, and maybe won't keep reading others of the books on the list. But they are sure still reading S. E. Hinton (and Mary Shelley).
       Mathematicians? Poets? Musical performers? How about classical composers? Mendelssohn, Mozart and Schubert all started doing their adult work while still in their teens.
       As for Walter Farley, he started writing The Black Stallion, the archetypal horse story, when he was 15, published it when he was 19. We have the book of course, as well as Carroll Ballard's lovely 1979 film.