I don't think "The Hobbit" is now
generally considered a teenagers' novel. It's probably just the decision of whoever stacked the shelves in that particular bookshop. Having bought plenty of books for my younger cousins in the last few years, I've found it can be a bit arbitrary what goes in the older kids' vs the "young adult" (teenager) sections. Sometimes this is no doubt the result of carelessness or even ignorance; other times there may be some logic behind it, even if we don't all agree with it.
Consider a sort of mirror-image case: a few years ago, Amazon.com
was selling COH as a children's book.
(Note that most of the Downers who commented defended this decision, on grounds which I happen to think are rather missing the point.)