The Messenger has a Lexile score of 720, making it appropriate for approximately fourth-grade level readers in terms of difficulty.
Commonsensemedia.org suggests the book for age ranges 9 and up, citing a lack of innocence in characters, questionable morality, and violent plants (trees in the Forest attack the characters; one dies). Parents and educators on the same website suggested the title for ages 12 and up.
This book does not provide any sort of resolution to The Giver, and one major issue that readers will face is the death of a major character. It also poses the issue of humanity destroying the world, which is one that might be difficult for students to relate to unless they are a little older. Approaching these topics effectively in class might make it easier to present the text to a slightly younger group.
In the area of Indiana where I teach, most schools use this series from 4th-6th grade.