“To my mind, YA is a subset of adult fiction, not of children’s fiction, and should be considered as having an entry reading age rather than an age *range*. The entry level is probably 13 or 14, but there is no upper level because the books are also for adults. Saying YA is 13-21, or 13-18 or whatever misses the point, because it suggests that the books are not for older adults, whereas I would say that in fact the core audience of people reading YA (and YA SFF in particular) are in fact 16-35. But this is only the core and the readership extends more broadly upward in age and down as well.” [From Podcast Coode Street Episode 20, second comment]

— Definition by Garth Nix. Love it!