But now there are also all the age-specific ones (young adult, middle grade, new adult, etc.). There has always been classification of books for different ages (children's books, books for teenagers, etc.), but now this seems to extend to the characters as well. In other words, when I was growing up the Dr. Dolittle books were classified as children's books, because they were for children to read. But the early books in the series had no young characters at all, and when the later books added a boy named Tommy Stubbins (probably for reader identification) he was a complete dud.
But with "young adult" books these days, there seems to be the assumption that the characters need to be "young adult" as well. I know somebody who just had a book rejected partly because the characters were supposedly the wrong age (by just a couple of years) to appeal to the intended audience.
I would not have liked this when I was young. Back then I read all sorts of books, mostly about adults. Mysteries, science fiction, etc. Robert Heinlein wrote a series of "juvenile" science fiction books, with young protagonists, but they were pretty thin compared to his other books.
On the other hand, I'm older now, and I still enjoy stories about young people if the stories are well done. For example, in recent movies I've enjoyed Hugo and Let the Right One In, and I never wished that the characters were older so I could identify with them better. Stevie One is about a young adult, but I've never classified it as YA since it's not particularly directed toward young readers. Coming-of-age stories have been around for a long tiime, and they've been enjoyed by readers of all ages, not just young ones. Good stories are good stories.
I read a lot about the lack of diversity in YA, but it's never about age-diversity. 🙂
The other thing that is driven by marketing rather than art is the emphasis on protagonists being likable and "relatable." This was one of the network's many complaints about the original pilot for Firefly, for example, that Mal needed to be lightened up. Whedon was able to steer things a bit in that direction without messing up the whole project, but it was a dumb idea to begun with.
Is Stephen Dedalus likable? (Hell, no). Is King Lear relatable? (Nope). And don't even get me started on the Westerns (Rooster Cogburn, Thomas Dunson, Ethan Edwards, etc.). Is Lisbeth Salander likable and relatable? Sherlock Holmes? Wolverine? Macbeth? Eve Harrington? Humbert Humbert? Charles Foster Kane?
I'm not advising against characters being likable and relatable. Write a good story, about characters who are interesting, facing interesting situations. Let the rest take care of itself.