Jean Piaget was a Swiss biologist and psychologist born in 1896 who developed the theory of cognitive development. He identified four stages of mental development: the sensorimotor stage from birth to age 2, the preoperational stage from ages 2 to 7, the concrete operational stage from ages 7 to 11, and the formal operational stage from ages 11 to adulthood. Piaget passed away in 1980 and made significant contributions to understanding how children's understanding of the world develops through these stages.