Eragon was written by a teenager with publishing connections. I don't know the story behind Divergent as well, but Wikipedia informs me that it was written while its author was in her senior year of college.

It's not so uncommon for writing, especially a first novel, to be published in its author's twenties -- Poe published several stories at that age, for example -- but teenage authors are a lot more unusual.

(I can't speak to their humor or lack thereof either, though -- my tastes in SF run a little more pretentious these days.)