Over half of today’s Young Adult readers are over the age of 18.
Given the adult popularity of childish fare such as "Star Wars" and the comic book superhero movies, this shouldn't be a surprise. In fact, academic discussion with millennial scholars is peppered with references to the Harry Potter characters.
Of course, at one time Tolkien's works were intended for the Young Adult reader, and they seem rather popular in the retirement communities that I visit.