What is an appropriate age to read this story?
There are classes at my high school that read this book. They make sure to give the appropriate background and historical context of the novel, such as the history of Apartheid and the class and race system that it established. The book does require a higher level of maturity, but it has such impact that even less mature students have gotten over initial squeamishness. If you are looking to use this in a classroom setting, then high school would be appropriate. If you are looking for reading material for an individual, then it may be appropriate for a mature middle schooler.
I'm not sure if there is ever an appropriate age for any novel: it really depends on the reader. Some readers are ready for more mature novels at an early age. However, that said, because of the strong themes in this novel, including abuse and racial tension, before I gave this book to anyone, I would make sure that they were mature enough to understand/handle it.