I am 30 and really enjoyed these books. It makes you question the whole 'do we have souls' and religious stuff... If I was buying for a child Id probably recommend 11-12 plus - I think at this age they would really enjoy it, although probably would miss the religious questions it raises!
I read them to my children when they were about ages 8 & 10 and they loved them. They have since (as teenagers) read them again themselves and love them all over again. I'm sure some of the deeper religious messages escaped them first time round, but that didn't spoil their enjoyment of the books.