From my local library's guidelines: "Although the Library does not categorize nor limit access to materials designated as ‘adult’ or ‘youth’, certain materials will be of particular interest to or appropriate for different age groups by virtue of the subject matter, level of complexity, etc., and will be designated as such. These designations are intended to provide a convenient mechanism for the arrangement of materials of particular interest to adults, children, young adults, teens, and adults working with children and are not intended to restrict use to a particular age group"

And from a document on the ALA website: "Equitable access to all library resources and services should not be abridged based on chronological age, apparent maturity, educational level, literacy skills, legal status, or through restrictive scheduling and use policies.

Libraries should not limit the selection and development of library resources simply because minors will have access to them. A library’s failure to acquire materials on the grounds that minors may be able to access those materials diminishes the credibility of the library in the community and restricts access for all library users."

https://www.ala.org/advocacy/intfreedom/librarybill/interpretations/minors

My local library had a sign in the YA section telling teens where to find the Sarah J Maas books in the adult section, presumably because they were asked this question to many times that it was easier to make a sign. They also had Arifureta (manga where the ml canonically has sex with his 1000+ yo loli wife [he's like 17]) in the ya section.

Libraries aren't places of censorship.