Two subjects I have to jump in on:
I'm so, SO glad that our library here interprets the line in the Library Bill of Rights about "no discimination of services by race, gender, age, etc" to mean that everybody gets them same card and can select books without interference from us staff. They may have interference from their parents, but that's the parents' decisions, not the library's.
My mother had imposed some age limits on certain books, but that had more to do with the power of the story than language or explicit scenes. I tried to start Of Mice and Men when I was 9 or so; it was withheld until I was 12 or 13, and it was still too much for me emotionally.
Subject two: I re-read The Scarlet Letter years after high school, at my own pace, and really enjoyed it for the most part. Many books I loathed in class, I've sat down and at least tried the first 40 or 50 pages, and ended up at least respecting the author's writing ability, if not liking the book.
In a perfect world, classrooms would have time to assign a book to just read through, understand what you can and look up the words you need to. Then, after you have the whole story in your mind, go back to the begining and analyze the bits that call for analysis.