(d) According to psychologist Jean Piaget, children progress through a series of four critical stages of cognitive development. Each stage is marked by shifts in how kids understand the world. Through his observations of his children, Piaget developed a stage theory of intellectual development that included four distinct stages: • The sensorimotor stage, from birth to age 2 • The preoperational stage, from age 2 to about age 7 • The concrete operational stage, from age 7 to 11 • The formal operational stage, which begins in adolescence and spans into adulthood.