"At four or five months ... evidence showing that some skills develop in children much earlier than Piaget's stages suggests. This is partly down to the limitations of 1950s technology.

Piaget found four major developmental stages (with many subdivisions). For the first year and a half or two years of life, infants are only aware of sensorimotor experience, and do not connect it ...

This period, known as the formal operational stage, is the fourth and final stage of Jean Piaget's theory of cognitive development. It begins at approximately age 12 and lasts into adulthood.