Schools have libraries, and those libraries are naturally segmented by age because of being attached to a specific school (elementary, middle, high). Kids have to know the book is there and seek it out, so that puts an additional buffer.
Books are also delivered via the classroom. Teachers, of course, exercise judgement there on what is age appropriate, both for content and for reading level. The bar should be higher there because students have little to no choice on the book.
The Magicians: The books were good, but the TV show really was in a class all its own. And it did away with using obscure words just because, that was annoying.
Game of Thrones: At this rate, ASOIAF is never getting done, so I'm by default giving it to the show for actually finishing the job.
Good Omens: The first season brought the book to life, but there wasn't source material beyond that. The second season did a great job fleshing out the characters and moving the story forward into the final season.