Full context:   Maria Montessori began her career working with mentally disabled children in 1897, then conducted observation and experimental research in elementary schools. She wrote The Discovery of the Child in 1950 which developed the Montessori method of education. She discussed four planes of development: birth to 6 years, 6 to 12, 12 to 18, and 18 to 24. The Montessori method now has three developmentally-meaningful age groups: 2–2.5 years, 2.5–6, and 6–12. She was working on human behavior in older children but only published lecture notes on the subject.