The NY Times is having a discussion over the right time for which books i.e. should a 6-year old read Harry Potter? There are a series of short essays from various people on the subject.
As a former children's bookseller, I would err on the side of waiting for certain books. You can read almost any book to any child at any time but many themes are possibly too big for some kids as presented in some books.
They could read them at 6 and then again at 12 and get something different. That would be okay (and to be expected) except that if the theme worries the child or causes upset at a younger age, was that worth it?
Those books will always be there; it's not like they go away.