I’m assuming that getting books banned from libraries requires them to be there in the first place (in most cases at least), so any arguments using examples age rating issues should rather focus on why those books got into a school library in the first place.
Surely the ones responsible don’t just blindly choose some books to fill the library without at least making sure they’re not as wildly inappropriate as some people like to say.

Fun trick I do with my children is to way things like “Oh yeah you don’t want that. It’s forbidden” if I want them to read more about it.

The Anarchist Cookbook is on the reading list