Jean Piaget was a Swiss psychologist who developed a theory of cognitive development with four distinct stages: sensorimotor, preoperational, concrete operational, and formal operational. The sensorimotor stage occurs from birth to age 2 as children learn through senses and movement. The preoperational stage is from ages 2 to 7 where children think concretely and learn through experiences. From ages 7 to 11 is the concrete operational stage, where logical and systematic thinking develops. The formal operational stage begins at age 12 through adulthood involving abstract reasoning and complex thinking.