Final answer:
Jean Piaget proposed that children enter the concrete operational stage of cognitive development around 7 years of age.
Step-by-step explanation:
Jean Piaget proposed that around 7 years of age, children enter the concrete operational stage of cognitive development. In this stage, children can think logically about real (concrete) events, understand mathematical operations, and grasp the concept of conservation. Piaget's theory of cognitive development consists of four stages: sensorimotor, preoperational, concrete operational, and formal operational.