What makes a book young adult?

After a lively debate at the office this morning about what makes a book fall into the category of Young Adult, I Googled the topic and found this interesting NY Times article which I had actually read before. The question posed in our debate, however, was, beyond the marketing choices of the publisher, what criteria specifically makes a book more suited towards kids than adults? My boss suggested that it was the simplicity of the prose that made a book Y.A., but there are many youth-geared books that are written in a more sophisticated style. I argued that it simply required a youth protagonist and to have been written specifically for a youth audience, style not being a criteria. However, there are a lot of titles that fall into a grey area of being universally read by both youth and adults. Harry Potter, for the most obvious example is marketed as Y.A. but more adults I know than kids have read and loved it. And then there is the even more undefinable coming-of-age fiction, which often times portrays youth but is geared towards adults (think Catcher in the Rye). So what really makes a book Y.A.? I'm not sure there is an answer beyond being shelved in the kids section at Barnes and Noble.