We'd need a rating between PG-13 and 18+. I find it strange that the USA doesn't have a rating between 13 and 17+. I wouldn't want a 13-year-old reading my writing because it does address some adult themes but a 16-year-old who's probably seen Game of Thrones isn't going to be corrupted by something less violent/dark then The Hunger Games. I have no extreme violence or graphic sex scenes.
Not all adult fiction is constant gratuitous sex and violence. I already have content warnings and mark anything more violent than a paper cut as mature.
Why should writers of adult fiction be blanket banned from the app when most people are reading on their phones? Why should authors be punished for not writing for 12-year-olds who could easily borrow Stephen King novels from their local library? There's a big difference between something rated PG13 and erotica. There's nothing wrong with erotica but can we stop forcing/expecting all writers aiming at an adult audience to write torture po*n with a thin veneer of story.