I think the problem is that "Young Adult" encompasses a wide level of maturity, and authors may tailor their works towards vastly different levels of maturity, that the quality can vary quite a bit. Some works are targeted for young readers, but are rather mature in their execution (well explained, well reasoned character motivations and plot, etc), while others bank on the possibly lower expectations of the young and inexperienced reader. I've picked up some horrendous young adult fiction before and read it (I'm looking at you, James Patterson!). That doesn't keep me from reading it, but it does make me wary...

I've found word of mouth and reviews to help out with that. But its not 100% proof alright.

Categories are about marketing. "Young Adult" categorization is part of a decision to market primarily to the YA demographic, not a content-based description that the book can "reasonably by read by" young adults in a way that doesn't apply to most other books in a bookstore.

It certainly makes sense from a marketing point of view, though even there it's possible to market a book to YA without making it a classification. I just don't think the distinction makes sense as a top-level classification.

The two purpose top-level classifications serve is (1) identifying where to look for retail customers, and (2) discussing distinct business market segments.

I think it makes pretty good sense from both of those perspectives.

It probably doesn't from the point of view that classifications should fit some neat content-description role, but that's never been the function that they actually serve except insofar as content descriptions have utility to the other purposes mentioned.

(2) could be solved by marketing the same book differently. I agree that (1) has value, and (2) has some value when classifying customers.

However, this thread started because many adult sci-fi and fantasy readers look down on YA books. That's the purpose for which I'd suggest that the distinction doesn't make much sense.

There's some really good YA genre fiction out there, but I think one of the defining aspects of the genre is it's written about teenage protagonists with a number of very teenage problems, which I find increasingly tedious the further I get into my twenties.

That said, Marissa Meyer's Lunar Chronicles (to pick a somewhat less known example) are pretty excellent sci-fi, if you can get past the annoying teenage romances of the protagonists.

Fair enough. I do very quickly skip stories that seem like a sitcom with fantasy elements thrown in. Then again, there are a lot of non-YA fantasy novels about broken people (teenage or adult) with highly non-fantastic problems; that seems to be a defining element of bad fiction for all ages, that it can't manage to escape the mundane. I have zero interest in reading about the everyday problems and situations of twentysomethings or thirtysomethings either.

Another good example of YA fantasy I've enjoyed that manages to dodge the trope: Diane Duane's Wizards series ("So you want to be a Wizard", "Deep Wizardry", "High Wizardry"), which seems to mostly stay away from typical teenager problems. The premise is not that novel, but in this case I found the execution quite satisfying, particularly in the second and third books. Without providing spoilers, I would note that the second book manages to avert one of the single most common tropes of YA fantasy novels (and of other series that avert it, most do so far later in a series). I'm also a big fan of "sufficiently descriptive language as magic" and "sufficiently analyzed magic", both of which these have quite a lot of.

(Note that I have not read the "updated" editions of these books that attempt to make some of the technology references less dated.)