Students in a Montessori setting are grouped according to their stage of development rather than their date of birth.
Maria Montessori observed, as we observe at home, that children learn a lot from each other. Younger siblings imitate older siblings. Older children love to care for and teach younger ones. Classes are designed around this fact.
By having a mix of ages in a classroom, children have a much greater opportunity to find a work partner who matches their current interest and ability. Older children become role models and leaders. They experience the joy of feeling competent and capable in their work and reinforce what they have learned the previous two years by passing on their knowledge to younger children. Younger children learn more quickly and naturally by imitating their peers and mentors.
Mixed ages also allows children to witness their own growth by measuring it against the other ages. They can easily see during their second and third years that they can do more than the children who have just joined their group. These new children can see the trajectory they’ll be on for the next three years, and mark their progress toward being the leaders in the class.
Socially and emotionally, mixed ages allow children to function in the manner most natural to them, and prevalent in society. Adults don’t separate into groups determined by birthdate. They work together in groups of all ages, with younger adults benefiting from the experience of older adults, and older adults benefiting from the new perspectives of younger ones. The Montessori classroom in this way functions most like how children will need to work when they are adults.