A Wrinkle in Time could probably be adapted for a variety of class ages. I do think that 5th grade is probably the youngest age that the book would work for, and a teacher's lesson plan would have to reflect the limits of the age group. The book would likely work best in grades 6-8 because they are experienced enough readers to tackle portions of the book on their own and old enough to begin limited discussions of the book's theme. With all of that said, my undergraduate "Children's Literature" class was assigned that book. Obviously, the professor's lesson plan was adapted for that particular age bracket and style of class, but it does serve as a good example of how a good teacher can adapt and tailor a lesson plan to the correct audience.
I have used the Enotes Wrinkle in Time lesson plan in a 7th grade reading class. It could be adapted for use in a variety of levels, from an advanced 5th grade reading class to a 9th grade English class. Basically, the lesson plan targets the middle school age groups.