Jean Piaget observed his children and developed a four-stage model of cognitive development. The four stages are: sensory-motor (birth to 2 years), preoperational (ages 2 to 4), concrete operations (ages 7 to 11), and formal operations (ages 11 to 15). At each stage, children progress in their ability to process new information and understand their environment, advancing from motor skills and symbolic thought to abstract reasoning and systematic planning.