I'm very bad at estimating reading levels for young people, though I do remember liking many young-adult and adult books when I was younger. Though I may not understand all the concepts, I was able to read and re-read those books as I aged and could grasp deeper meanings in the text. Could Watership Down be one of those books to come back to in years to come for a child? Or is it plain too old for an 11-year-old reader?