Montessori observed four distinct periods, or "planes", in human development, extending from birth to 6 years, from 6 to 12, from 12 to 18, and from 18 to 24.
Montessori observed four distinct periods, or "planes", in human development, extending from birth to 6 years, from 6 to 12, from 12 to 18, and from 18 to 24. She saw different characteristics, learning modes, and developmental imperatives active in each of these planes and called for educational approaches specific to each period.[34][35]