Jean Piaget is considered the foremost theorist on cognitive development. He believed that intelligence ensures balance between a person and their environment through continuous processes of assimilation and accommodation. Piaget described four main periods of cognitive development and thought that intellectual ability differs at each stage. Elementary school children encounter developmental milestones as they progress from egocentric thinking to more mature perspectives through decentration, developing skills sequentially within certain timeframes. Reasoning remains immature in children, who learn new abilities rapidly until age 8 when skill acquisition levels off.