The book in question was Dune, which despite some valid points raised, I'm still not willing to think of as being particularly YA. I would confidently assert Frank Herbert didn't think of his book in those terms, either, though authorial intent is often secondary in these things.
At the end of the conversation, I'm not sure I learned anything about where those boundaries are, or even if they exist, but it was interesting nonetheless. I could, if I were cynical, adopt the position that a YA designation - much like NA or Literary - is more of a marketing standpoint than anything else. A way to determine what part of the bookstore or library (or page on Amazon, I suppose) the book should be shelved in. But even though there are sci-fi, mystery, and fantasy books in the YA section of my local library, I think it's about more than who gets to read the book.
My confusion wasn't helped any by another conversation elsewhere - I don't remember exactly where - about whether horror is a separate genre, or more something like YA. I would make the argument that horror is a genre, as it has its own conventions and trappings much like other genres. It's no different than romance or Western. Of course, my local library would seem not to agree. The library has a mystery section, and a western section, and a science fiction section... but if I want to pick up a book by King or Koontz I have to wander into the general fiction section.
(Though maybe that's more about the short-sightedness of the people who came up with the system than anything else. Urban Fantasy gets dumped into science fiction, interestingly enough, but that probably wasn't even something under consideration the last time library call numbers were assigned.)
And then to further muddle the heck out of things, as I was wandering in the library yesterday looking for a book to read, and not finding anything, I remembered The Last Policeman being on my list of books to check out. As it is an end-of-the-world story, I expected it to be in the science fiction section.
It wasn't there.
Instead it was under mystery. Which, to be fair, it is. A homicide, in fact, but set at the end of the world (not an exaggeration, and although I just started it this morning based on the pages I covered it's going to be a good book). So it's just the set pieces that are science fiction, while the plot is all crime fiction. But then again, so are more than a couple of Asimov's Robot books. They just happen to use a shiny future, with robots, rather than a "we're all going to die in 6 months" bleak future.
If the The Last Policeman had a robot, would it then be reshelved? If Asimov had not included robots, would they be reshelved? If Paul in Dune had spent more time brooding in his room, would I be more willing to accept it as YA? Maybe. Maybe not. Honestly, at this point I have no idea. Dune still looks sci-fi to me, more than anything else it is. Space travel, exotic planets, strange life-forms. That says sci-fi to me, and it says that as its primary identity. Not YA.
In the meantime, I'm going to settle for organizing my books alphabetically.