I depends on what bothers your child. I agree with the estimate that they get dark about book 4. Keep in mind that J.K. Roweling deliberately aged the maturity books unlike other authors. Excepting that her original audience would keep reading. So with the first few books fit the 8-12 year old reading level & interest. The later books are defiantly teen with more complex ideas and a lot darker, with teen themes. I'm not sure a 6 year old is going to "get" why Harry is so moody in book 5. I read the books to/with my son when he was in early elementary, and then he read them to himself when he was 11.
