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Equine I read this when I was fifteen or sixteen, and really enjoyed it. Depends more on the individual in question than anything else.
Katie I would recommend this book for adults. First, to really understand the story, you have to have some background about WWII and Nazi Germany. Also, It has some very mature themes. Second, the book is very detailed and requires persistence in reading it. I think the earliest age this book could be read would be junior or senior year of high school.
Kelley Jones I would say anyone 15+ with an interest in history. Can be slow, so adults would be more likely to enjoy it. You definitely need a solid understanding of the players in world war two