Why Mixed Age Groups?
If classroom equipment is to be challenging enough to provoke a learning response, it must be properly matched to the standard which an individual child has already developed in his past experience. This experience is so varied that the most satisfying choice can usually be made only by the child himself. The Montessori classroom offers him the opportunity to choose from a wide variety of graded materials. The child can grow as his interests lead him from one level of complexity to another. Having children ages two years, nine months through six together permits the younger children a graded series of models for imitation, and the older ones an opportunity to reinforce their own knowledge by helping the younger ones.
Also see The Role of the Teacher, Behavior of the Children, Why Mixed Age Groups? and the Non-Competitive Atmosphere