I've always liked books and I've always liked writing ever since I was able to read and write, but when I was about 16 I read Jonathan Franzen's The Corrections. I'm not sure why, but something about that book made me want to get serious about writing. I don't know that it would affect anybody else in the same way, but it moved me in a way that nothing else had before. There was also an epiphany I had that was just kind of like, Oh, somebody is actually taking this writing thing seriously.
I've written almost every day since.
Also, Brief Interviews With Hideous Men is solely responsible for interesting me in writing short fiction.
EDIT: After re-reading the first post, if you're looking for novels written by exceptionally young people, Jenn Crowell's Necessary Madness is one of the most impressive books I've read recently. She started it when she was 17.