As students physically grow throughout their education they are gradually becoming capable of more. Arnold Gesell, a famous pediatrician, composed a theory on physical development. He determined that individuals physically develop in a specific order, but not at a specific time. One child may walk at nine months, where another child doesn't take their first steps until twelve months. Both children are perfectly normal according to Gesell.
Cognitive development is what the individual knows, and how they acquire their information.
Piaget's theory of cognitive development is very logical. He set the stages of cognitive development up into four periods. Piaget's four periods were called the sensory-motor, preoperational thought, concrete operations, and formal operations periods. According to Piaget the third grade students I worked with at Howe School were part of the concrete operations period. Piaget recognized that children pass through his stages at different rates, and he therefore attached little importance to the ages associated with them.