Eleanor and Park (Rainbow Rowell)

Eleanor and Park is yet another example of a book party to which I'm too late for a substantive review. I mean, it came out a year ago, and every young-adult-reading-adult I know who's read it has RAVED, quite rightly. And if you, dear reader, are a YA-reading-adult, you've already heard of it. And I've nothing new to add to the accolades: it's wonderful. Beautifully written, heartbreaking, heart-lifting, yes-this-is-exactly-what-it's-like-falling-in-love-ing.

What I have instead of a review, then, is a question, one that makes me wish I'd done more blog promotion and whatnot so that I had an actual community of commenting readers, because I am likely just asking to the ether: Why is this a young adult novel?
Genre, and delineations thereof, are tricky--especially those based on the assumed age of the audience. And the YA Crossover phenomenon is well-documented: Twilight, The Hunger Games, The Book Thief, and so forth. Indeed, these days some "young adult" titles are picked up by publishers specifically to become crossovers. I don't think Eleanor and Park is one of the latter, and it falls under the vague general definition of YA in that the main characters are teenagers. But to me, speaking only to myself, it feels like a retrospective adult novel--that the third-person voice is that of an older person looking back on first love and adolescent trauma, not that of a teenager experiencing it firsthand. This isn't an insult, or even a quibble: I loved this book, I'm so sad I have to give it back to the library at the end of the month. I just want someone to sit me down and explain. Please?