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. Children from 3-6 years of age are together in the Children's House; 6-9 year olds share the lower elementary and the upper elementary is made up of 9-12 year olds. Because the work is individual, children progress at their own pace; there is cooperation rather than competition between the ages.