At the NY Times:
The fact that at the moment the distinction is being made, a young adult, as opposed to an adult, is the one reading it. In other words, I don’t entirely believe in the distinction. A great book is a great book, and it’s impossible to say what part of a person is going to connect to it. Age and experience aren’t always among the most relevant factors.
Personally, I think the distinction lies more in the marketing than in who's reading it. And that in YA, the characters are usually teenagers who are dealing with a lot of the Big Life Stuff for the first time (first heartbreak, first death, first betrayal, first murder investigation, etc...), but as we all know, YA does seem to be defined differently by pretty much everyone you ask.
For me, I don't care what the book is labelled, as long as it's good. (Which, actually, Cashore says earlier on in the interview.)