Swiss psychologist Jean Piaget (1896-1990), created a cognitive-developmental stage theory that described how children's ways of thinking developed as they interacted with the world around them. Infants and young children understand the world much differently than adults do, and as they play and explore, their mind learns how to think in ways that better fit with reality.
Piaget's theory has four stages: sensorimotor, preoperational, concrete operational, and formal operational. During the sensorimotor stage, which often lasts from birth to age two, children are just beginning to learn how to learn. Though language development, and thus thought, does begin during this time, the more major tasks occurring during this period involve children figuring out how to make use of their bodies. They do this by experiencing everything with their five senses, hence "sensory," and by learning to crawl and then walk, point and then grasp, hence, "motor."
During the preoperational stage, which often lasts from ages two though seven, children start to use mental symbols to understand and to interact with the world, and they begin to learn language and to engage in pretend play. In the concrete operational stage that follows, lasting from ages seven through eleven, children gain the ability to think logically to solve problems and to organize information they learn. However, they remain limited to considering only concrete, not abstract, information because at this stage the capability for abstract thought isn't well developed yet. Finally, during the formal operational stage, which often lasts from age eleven on, adolescents learn how to think more abstractly to solve problems and to think symbolically, e.g., about things that aren't really there concretely in front of them. As is the case with Erikson and Kohlberg, Piaget's ideas will be developed in greater depth in future documents.