Stages of Cognitive Development
What we know about human cognitive development is due in large part to Jean Piaget, a Swiss clinical psychologist who pioneered work in child development. His epistemological view of the growth and development of intelligence from infancy to adulthood focuses on six basic assumptions, the first three being that children are active and motivated learners, construct knowledge from their experiences, (6) and will learn through processes he identified as assimilation and accommodation.
Assimilation involves dealing with new objects or events in ways that are consistent with currently existing schemes. Accommodation takes place when new information does not fit into existing schemes, thus requiring schemes to be adjusted or accommodated.
Piaget also assumed that children require interaction with physical and social environments as an essential component of cognitive development, and that progress toward increasingly (7) complex thought is aided by what he called equilibration, the moving back and forth between states of balance and imbalance. Piaget believed that it is the desire for balance that forces children to construct new schemes or accommodate existing ones.
(8)  Piaget’s sixth assumption was that the process of cognitive development is stage-like in nature.  The first is the sensory motor stage and occurs from birth to approximately the age of 2.  Accordingly, his theory holds that children move through four distinct stages.  During this time, children are in the process of acquiring knowledge about objects and how they are manipulated.  They also begin to understand how one thing affects another and to develop ideas about time and space.
Children are characteristically egocentric during this stage, meaning they are unable to consider anyone else’s needs, wants, or interests. Most of their time is spent acquiring information about themselves, their world, and the people in it.
The preoperational stage usually occurs between ages 2 and 7, during which a child’s thought processes and vocabulary expand and develop. Children continue to believe everyone shares their point of view, but gradually become able to place the center of attention on others. During this period, children also become capable of symbolism in which one thing is taken to stand for another.
(9) From about age 7 to 11, children go through the concrete operational stage and are able to think logically about an object if they are able to manipulate it. They gradually learn that objects are not always as they appear and also begin to realize that a change in appearance does not necessarily mean a change in substance.
(10) The formal operational stage begins at age 14, during which children develop organized systems of adult intelligence. They are able to reason beyond concrete reality to consider alternative possibilities and to formulate logical thoughts using symbols and information that does not necessarily refer to real-world objects or events.