One of the hallmarks of the Montessori method is that children of mixed ages work together in the same class. Age groupings are based on developmental planes which are birth to 3, 3-6, 6-12, 12-18, 18 to adulthood., Because the work is individualized children progress at their own pace; there is cooperation rather than competition between the ages. (Haines)

“…We consider that children of different ages should work together, and in practice we have determinedthat these children should be of three different ages - of one year's difference between, for example: classes of three, four and five years or classes of six, seven and eight years, and that boys and girls should be together. Having three years of difference in one class enables the younger children to be helped through imitation or otherwise of the older children, and as it often happens that the older children give real lessons to the younger ones, it also gives an occasion to the older ones to learn to sympathize with the younger ones by realizing how one reacts when one has not yet reached the mental development of the older age... As we do not force the children to this reciprocal help it results in a form of social relationship...” (Montessori, Maria, Communications, “One can never hear it too often” 1980, 4, p. 23)