I think that this book would be more appropriate for late elementary and middle school students. The story does interest girls and boys alike. It depends on the reading level of your students more than the grade level, because this book will not make them feel babyish. For example, I think this book could be used in 8th and 9th grade or high school if you are trying to find a literature circle book that is high-interest and low-level. It is about a fourth grade reading level, and any high school students actually read at about that level.
Walk Two Moons is a story by Sharon Creech which features a 13 year old girl, Salamanca Tree Hiddle, as a main character.
This story would therefore be appropriate for students slightly below and slightly above that age group, making it very good for students ages 12-14.
Although the plot is very good and the narration is engaging, students who are 15 and above may not want to identify themselves with a 13 year old girl, and students who are in the 4th and 5th grade (10-11) may not grasp some concepts that are inherent to adolescents.
However, the fact that Sal has plenty of things to say and mysteries to share will engage undoubtedly just about any student who enjoys a good reading and appreciates good story-telling and suspense.