In Jean Piaget's Cognitive Development Theory, there are four stages, which would show how complicated children's thoughts and understanding are.
Children from birth to 2 years old, they are in the sensorimotor stage.
Children from 2 years old to 7 years old, they are in the preoperational stage.
Children from 7 years old to 11 years old (some resources would stretch until 12 years old), they are in the concrete operational stage.
Children from 11 (or 12) years old until they reach adulthood, they are in the formal operational stage.
