Stephanie,
Not sure if you/they were looking for nonfiction/memoirs, or fiction. S.E. Hinton was a teenager when she wrote The Outsiders, as was Christopher Paolini when he started the Eragon series (I'm sure Carl Hiaasen being a family friend helped getting it published). Gordon Korman got his first book, This Can't Be Happening at Macdonald Hall, as a teenager in Canada. Kody Keplinger wrote The Duff and Shut Out before turning 20. As for memoirs, there's always Malala's.
Thanks for the link to the interview!
Cathy Andronik
(Retired) Brien McMahon HS
Norwalk