As anyone who has ever wandered from section to section in a bookstore (or music store, or, heck in their email foldering system) knows the subdivisions, or genres as people often refer to them are just a starting point so that if you know you like a certain kind of thing, you can start over there on the left rather than having to comb the shelves for hours or until you give up and pick one because it’s purple.
But, of course, there can be issues with such subdivisions. Someone, or even a committee of someones, decided that this book was this because of this, but really you read the description or heard a recommendation from your friend and you thought it was this. And sure, most people expect this and they have nice info people (hopefully) and/or kiosks where you can look it up and eventually find it. I’ve told people the story of searching for _Seabiscuit_ and checking “Biography” and “Animals” and “Celebrity Biographies” before asking and discovering it was in the “Sports” section (which makes sense, don’t get me wrong, but I think all of mine were not crazy also).
So, with the rise of young adult as a category becoming less things that people think will be good for young adults* to read, and a broader description of young adult as a category of books where the main protagonist(s) are young adults. Period. How is this different than before? Well, the biggest difference is that books that previously might have been shelved in the adult section (which I think is usually called "Fiction", unless of course it’s a mystery, sci-fi or romance) due to adult themes (abuse, sex, rape, drug use) are now being shelved in young adult.
Now, when I say shelved, certainly some bookstores and libraries are making their own choices, I’m just speaking at the level of broad categorization.
Some examples of books that based on this classification are young adult:
Romeo and Juliet
The Adventures of Huckleberry Finn
To Kill a Mockingbird
Catcher in the Rye
Now, I just want to point out, that I read all of these books in school, so clearly the idea that these books were appropriate for school aged children is not new. And no, I do not mean to suggest that Shakespeare intended his play to be only for young adults.
Because this brings me to my next point, there is no upper age limit (or lower for that matter) for readers of young adult. Nor is there intended to be. The new classification is intended, just like that of fantasy or romance, to assist readers (of all ages) who are looking for a certain type of book to read.
So, when I hear someone (who does not deserve the linkage) suggesting that signage further breaking down the young adult section is a terrible sign heralding the death of publishing, I beg to differ. Because, as you may be able to guess, if the broad category of young adult only really tells you the age of the protagonist, well, then, what happens if you like reading about teens but only mysteries, or romance, or paranorma? I’ve seen quite a few bookstores segment the YA section so you can better find them. And if that causes further confusion, well, thank god for info people and kiosks.
*In the case of books, young adults generally refers to 13-19, high schoolers in particular. Although there is overlap sometimes with the middle grade (11-13 - ie middle school) and adult, and there is some discussion of a post-YA category targeting the 19-22 range.