Reply #23: That's functionally the same as censorship, and demands the same questions

[Orrex]

Who governs the access to these books? On what basis? How might their governing decisions contested? To whom?

Excepting such guidelines as "age of majority" and the like (which is itself, in this context, a dubious and artificial threshold), books must be available to any who wish to read them.

If certain books contain mature themes, sexuality, or violence, then IMO parents have the authority to limit their own children's access to those books, but that's not a ban; the parents are, in effect, making a decision on their own behalf, since they are empowered/required to make decisions about their own children's education and emotional growth.