Childhood's end?

John Clute recently had a write up on Shadow Bridge.

I really liked Shadow Bridge. It should come as no surprise to anyone who knows me that nested stories are something I find really cool. I think Shadow Bridge delivers on that. I loved the bridge world, and how the image of it going on for ever and ever, curling off sometimes, and constantly connecting looked to my imagination. I didn't want the place to have to work with a conventional world look and feel.

I liked the characters too. There's a blend of near-real culture that's sort of real world and sort of not. I've seen is so frequently done with western cultures, so the emergence of pseudo-Asian or Polynesian culture is really fun. There's fox women, island fishing cultures, djinn and shadow puppetry. It's a good thing to read.

Clute saw it as a YA novel, which I wasn't quite sure about. It's certainly got coming of age elements. There's not as sharp a line in what makes a YA novel. I can point to things that certainly aren't, but someone's reading level is a rather subjective thing. Some books are intentionally targeted at younger readers, and it's evident when they are, but I didn't get as much of a feel of that from Shadow Bridge.

Where does YA end, and adult fiction begin? What are YA books that, as adults, we ought to read?