The librarians are responsible for curating books and assigning them to the appropriate sections. That said librarians are human and mistakes can happen, if you believe a book is inappropriately categorized speak to a librarian about it (in a respectful manner please) and they will reevaluate the book.
However, while a certain book may generally be appropriate for a certain age range, not every book is appropriate for every reader. While the librarians can offer guidelines, in the end it is the parents responsibility to ensure their child is only checking out materials appropriate for them. This duty cannot be off loaded onto the librarians.