Movie theaters are privately owned businesses that can choose to show only children's movies or westerns. Similarly, video stores can decide not to rent certain movies to anyone under the age of 18. Libraries must meet the diverse needs of everyone in their communities. They cannot overrule the rights and responsibilities of individuals by deciding who does or doesn't have access to library materials. Most libraries provide movie reviews and ratings for parents to use these in guiding their children's library use.
Children mature at different rates.
They have different backgrounds and interests. And they have different reading levels and abilities. Or parents may feel a particular library book is inappropriate for their daughter, while the same book may be a favorite of her classmate's family. These factors make it impossible for librarians to set any criteria for restricting use based on age alone. To do so would keep others who want and need materials from having access to them.