In So You Want to Read YA? at Stacked editor Andrew Carre says, "Young adult books are about adolescence, not for adolescents." This is a thought-provoking statement, though probably not a hardcore definition of YA literature, since if YA literature is merely about adolescence, why have a separate YA category? Especially since Carre's post is all about about encouraging adults to read YA. His point is that adults shouldn't let that YA category label keep them away from YA books.
If young adult books are merely about adolescence, why not just publish them within general fiction for anyone to read, which is where many books about adolescence are published? That is, after all, where those YA classics Catcher in the Rye and To Kill a Mockingbird first found homes.
On a related note (really), I am reading Swamplandia!, which, so far, seems like YA to me. That's not how it was published/marketed.