Dr Montessori outlined four consecutive planes, or stages, of development from birth to maturity, each plane spanning approximately six-years. At each plane of development children and young people display intellectual powers, social orientations and creative potential unique to that stage. Each plane is characterised by the way children in that plane learn, building on the achievements of the plane before and preparing for the one to follow. The timing and nature of the transition between planes vary from individual to individual.
- The first plane of development is the period from birth to, approximately, age six. During this stage children are sensory explorers, learning to become functionally independent in their immediate environment and community. Children at this stage construct their own intellect by absorbing every aspect of their environment, language and culture.
- The second plane of development is the period from, approximately, six to twelve years. The developmental focus of this period is intellectual independence, hand in hand with the development of ethics and social responsibility. During this stage children become conceptual explorers. They use reasoning, abstract thought and imagination to explore and develop their understanding of the world.
- From age twelve to eighteen young people become humanistic explorers seeking to understand their place in society, and to contribute to society. They have a huge capacity for creative expression, and their style of learning becomes more practical and experiential, an approach they use to explore previously introduced concepts in more depth and in real-life contexts.
- From eighteen to twenty-four young adults develop specialist knowledge and skills, preparing them to take their place in the world and to establish social and economic independence.
For each plane of development there is a specific Montessori learning environment. Montessori environments for each plane maintain distinctive Montessori characteristics, including freedom of choice and movement, and an emphasis on independent exploration and self-directed learning. At the same time the design of each environment is customised to the specific needs, interests and potential of each developmental stage.
Within each plane of development there are periods during which children and young people display intense interest in a particular activity or aspect of the environment. These periods were called by Dr Montessori sensitive periods for learning, especially in the context of early childhood. The sensitive period for language, for example, is active during the first plane of development from birth to six years. This sensitive period provides a window of opportunity that enables children to learn language with ease and enjoyment. If, for any reason, a child does not learn to speak during this time, the sensitive period disappears and the learning of language requires much greater effort. The particular learning sensitivities and needs of children at each stage of development are reflected in the design of the Montessori environment and in the resources and activities for that stage of development.