When I worked in a school library we had age restrictions on a lot of our books and parents would have to provide written agreement for most things which were thought to contain 'adult themes'. That included Pullman and Rowling books - not because of religious concerns but because they are quite scary!
Oh, I bet. Could you give some examples of other books that had similar restrictions on them? Did you, for example, age mark The Lion, The Witch and the Wardrobe?
You had to be 9 to read Narnia (same age as the first 2 Potters) - I don't remember anyone asking for Narnia earlier - we had a lot of requests for Potter.
I do recall having to have agreement letters for Michael Morpurgo and Jacqueline Wilson - and 1 boy wanting us to buy a copy of "The Cider House Rules."