Catherine I read it in 12th grade for my Literary Explorations class. However, there is no "mature" content, so you could read it at, say, twelve years of age. Most twelve year-olds wouldn't get much of the messages in it, though.
Daniel Kuntz I will be reading this book next week for my honours English class (I am in 8th grade) so I assume it is age-appropriate for students at least in 9th grade.
Terry Bourbon My son is a Freshman in HS, and is reading it now. I took his book to read it. At least one of his older sisters read it in high school as well.
Jesse Cheshire There is no 'grade' for this book. If I were the literary professor or high school English teacher I would have it read at about 9th grade with an essay written, and then repeated in 12th to evaluate how each student's understanding of the concepts developed.
Brianna McShea I read this in 5th grade. I can honestly say this was my favorite book throughout my entire educational career.