It's not about age. I'm in a good few reader and author groups on FB, and the median age is closer to 40 than 20.
The most popular books are series of undemanding mass-market fiction, usually with some repeating branding in the titles and/or cover art that makes it clear they're related.
People don't just read them, they collect them. And then post photos of their collections. And also post how well how they're doing on their hundreds-of-books-a-year reading challenge.
It's a world I never knew existed, with a very unfamiliar culture.
I think it's really more about conspicuous consumption than reading for pleasure or enjoyment.
Gamification has made it compulsive.