Two interviews.

One very brief with Charles De Lint at The Star:

"I don't write differently for different age groups," he says. "In fact, I never even thought I was writing Young Adult stories until I was approached by Viking. They wanted to do a collection of my Young Adult short stories. And I told them I didn't have any Young Adults stories and didn't have time to write any.

"They said, `Well, you already have.' And they pulled out a list of all the stories I'd written with younger protagonists.

"So that's kind of the way I look at it now. It's the age of the characters that defines the genre."

I think it's a bit more complex than that, but it seems like there are as many definitions of YA as there are writers and readers of YA.  So there you go.

And one less brief with E. Lockhart at SLJ:

Frankie Landau-Banks definitely had a feminist slant. What do you hope young readers will take away from her?
I tried to make Frankie both an admirable character and a bit of a psycho. She's on the edge of losing her mind. She's a bit unbalanced in the urgency of her ambition. And yet–that’s what heroes are like, yes? I want readers to argue about her.

I'm so glad she said that.  Because I felt that Frankie was a little unhinged.  Awesome, but unhinged.