Piaget's theory identifies four stages of cognitive 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 15. During each stage, children build their understanding of the world through physical interactions and experiences that help them progress from concrete to more abstract thought. Piaget's theory emphasizes how hands-on learning experiences are critical for supporting children's cognitive development as their mental models become more complex over time.