The ALA extends that policy to videos and the internet.
The ALA's policy does read that material in the library shall not be selectively denied to patrons based on age, gender, race, etc. But as far as the internet goes, relating to public libraries that get federal funding, they MUST comply with the Child Internet Protection Act.
And on a practical level, many libraries do monitor minor's use of the internet. Many academic libraries, which as a rule do not censor the internet because their services are by definition meant for adults, prohibit minors from using the internet altogether.