At Cabot Montessori School we have two classes: Early Primary and Primary. The Early Primary class has the younger students with ages ranging between 2 and just under 4 years old. The Primary class has the older students ranging in age between 3 and 5 years old.
Multi-age grouping benefits not only the younger children who look up to and imitate the older children but also the older children benefit from teaching the younger children what they know by reinforcing their own understanding of the material. They are able to recognize gaps in their own knowledge simply through the process of explaining the lesson to someone else. They are also able to develop their leadership skills in a way they are not able to when they are in traditional same-age classroom. They model the acceptable behaviors that were modeled for them.
For further reading on the topic of multi-age groupings the articles Multi-Age Grouping: Observation + Imitation = Learning and Three-Year Multi-Age Grouping in the Montessori Classroom provide a lot of insight on multi-age groupings in the Montessori approach.
Our schools show that children of different ages help one another. There are many things which no teacher can convey to a child of three, but a child of five can do it with ease.
Maria Montessori