After observing children during a series of interviews during the 1920s, noted Swiss psychologist and philosopher Jean Piaget, outlined the stages of childhood to explain human intellectual development. From his observations, Piaget lists milestones or benchmarks to show childhood intellectual growth within each stage. Piaget's theory uses age to isolate the stages, but the cognitive growth of some children doesn't necessarily align with the ages listed in Piaget's research. Some children advance quickly, while others move more slowly through the stages.
Piaget labeled the time between birth and 24 months the "sensorimotor intelligence" period, and he then divided this into 6 separate categories. Infants during the first two stages -- from birth through the first month and then from the first month through the fourth -- discover the body through testing reflexes and adapting the reflexes to the environment. The third and fourth substages explore responses to the people and objects in the child's environment, including becoming aware of parents from 4 through 8 months. The basics of movement coordination also happen during the eighth month to the first year. The last two substages in the sensorimotor stage include exploration of actions from 12 to 18 months, and then making symbolic representations of these actions through trail and error from 18 through 24 months.
The preoperational period from 2 to 7 years describes the way children develop the use of symbols, according to Piaget. Symbols include the child's ability to use and make meaning of images, gestures and words to describe events, people and objects. The child rapidly develops language during this five-year period, and children learn to play using the imagination to create stories and situations. Piaget divided this period into two formal substages. These additions transformed Piaget's theory into six formal stages from the original four presented in the psychologist's original writings. Piaget labeled the "preconceptual stage" as the time from 2 to 4 years. The second substage takes place during ages 6 or 7, and Piaget labeled the formal period the "intuitive" stage. The first substage explores concepts and the second makes order and sense of the new concepts.
The concrete operational stage of Piaget's theory occurs between the ages of 7 and 11 years. Children in this stage develop the ability to think logically about concrete events, but have yet to use abstract thinking in a sophisticated way about non-concrete events. Piaget's term "operation" describes a logical thinking framework. This stage marks a critical period for children in building skills for adult-type intellectual operations, including deductive thinking -- the intellectual method of taking the parts of something to create an overall idea. Children also have the ability to place items into groups and subgroups, and they think about how the characteristics of the items allow for these groupings.
Children develop more formal systems of thinking and use rules to think formally about ideas from age 11 to approximately 15 years, according to Piaget's cognitive stages of childhood. During the formal operations stage children develop the ability to think in abstracts. Some childhood educators divide the formal operations stage into two separate subdivisions -- early formal operational thought and late formal operational thought. The child's sophistication in thinking marks the difference between the two subdivisions during this formal operations stage.