I wonder if this ever more specific age categorisation is the result of having so much being published online? I mean, it's not as though there are actually more readers now ... are there? Or maybe it's so that all those that were YA, but have gotten older, will know what they should be reading now? I'm waiting for PA (Proper Adult - we can't pretend any longer), MA (Middle Aged - don't call me old) and AA (Aging Adult - well over the hill, and loving it) categories
Having them selves on, it seems to me. As far as I can tell people will read what they like, whatever age group is given by a publisher. Young kids have long read stuff that was theoretically too old for them, and enjoyed it and learned lots along the way. And there are many older readers that love kids and YA fiction for the simple escapism with minimal effort they offer.