Jean Piaget's theory of cognitive development proposes that children progress through four distinct stages as their mental abilities develop: sensorimotor (birth to age 2), preoperational (ages 2 to 7), concrete operational (ages 7 to 11), and formal operational (ages 11 and up). At each stage, children demonstrate new cognitive abilities as they interact with their environment. Piaget's theory emphasizes that children are active learners and discover and understand concepts through hands-on experiences.