Rambling about book categorization

I've started wondering how books get categorized by libraries and bookstores. The book that started me wondering is the one I posted about yesterday: The Blue Sword by Robin McKinley.

When I bought the book, I distinctly remember that I found the book on the shelf in the Science-Fiction/Fantasy section. However, when I've seen The Blue Sword on the shelf in the store in the past months it's been shelved under Teen Books. I've also found out via LibraryThing that this book is a Newberry Honor book, suggesting that it was always intended for young adults. The Ace edition which I have has nothing about any of that that I can find on it, either in the reviews, the back cover or the front cover.

That's not the only example I can think of either. Orson Scott Card's Ender's Game has been shelved in both Teen and Science Fiction, as has Tolkien's The Hobbit (Childrens' and Fantasy). When I'm selling the book, I usually direct people to the editions in Fantasy, as it has (depending on the edition) either got Tolkien's own illustrations, or is the Alan Lee illustrated version.

Library cataloging gets even more confusing. Mercedes Lackey's Heralds of Valdemar trilogy (Arrows of the Queen, Arrow's Flight and Arrow's Fall) were shelved in the young adult section of my local libraries, but in the bookstores they're classed as fantasy. The same thing is true of a lot of the more recent Star Wars novels. For some reason, although the majority of the earlier Star Wars novels are in the regular fiction shelves, most of the New Jedi Order books are to be found in the Childrens' department at the library. Every bookstore I've been in has them shelved as part of the Science Fiction section.

Even in the Childrens/Teen books there's a fair amount of variation. The Trickster books by Tamora Pierce are definitely for older teens, while the Immortals series and the Keladry books are, in my opinion at least, more suited to the true childrens shelves, yet they're still shelved as teen books (but not always with the other Tamora Pierce books). I found the earlier ones (Alanna and the Immortals Quartets) in the childrens department of my local library, so it seems that they've been moved upwards.

And then there's the Heinlein novels. Some of those, although categorized as Science Fiction seem more to be suited to the Childrens/Teen sections. (Of course, I'm not talking about Time Enough For Love or any of the ones like that. I'm thinking more of The Rolling Stones and the like) I'd love to know what their original audience was. Was it science fiction, or were they originally written for children?

What is it that gets a book categorized as one or the other? I'd like to know, simply to make it easier to make recommendations for people. I'm always hesitant when recommending to teens in case their parents won't approve of my suggestions.