That's a hard question to answer. Technically, this book is suitable for children as young as 6 years old but I doubt that most would completely understand it at that age. One of my professors once told me that this book should be read 3 times in one's life: as a child, as a young adult and as an older adult. I think he said so because as we grow older our perspective changes, and thus we become able to appreciate things in different ways.
I'd say that teenagers and adults would enjoy this book the most.