I asked this same question a year or two ago and also got downvoted for it. I even suggested that books have a sort of rating system on them like movies do. People did not like that idea. I think people who like YA books get upset when other people don’t like them because it makes them feel like they like something meant for kids and somehow that is a bad reflection on them or something idk. Personally I don’t really like YA books when I’m reading them for myself. When I sit down to watch a movie I don’t watch a movie meant for kids because it just isn’t as entertaining to me. Same thing with books. But unlike with movies you don’t really always know if it is a book meant for kids or adults. Same exact thing has happened to me many times asking for recommendations.