One of the hallmarks of the Montessori philosophy is multi-age groupings. It is a joy to watch the children as they go through their day being helpful and kind to each other. The older children in the classroom take pride in being leaders in the class. The younger children learn so much from the older students.
The older children are such good role models. The leadership skills and compassion they acquire in the Montessori environment are unsurpassed. It is very much like the real world where adults interact with people of all ages and personalities. Multi-aged groupings also allow the children to reinforce the academic concepts they have already learned by teaching others.
The lessons the younger children learn by watching their mentors is invaluable. They watch how the older ones behave in the classroom, how they choose work, and how they interact with the other children. They also learn academic concepts just from observing!