Jean Piaget was a Swiss psychologist and biologist that studied the cognitive and intellectual development of children. Children are classified into one of four stages of development including the sensorimotor stage, the preoperational stage, the concrete operational stage, and the formal operational stage. Piaget believed that children only advance to the next stage when they have had a sufficient number of experiences within each stage.

From their birth to approximately two years of age, children are considered to be in the sensorimotor stage. During this stage, knowledge is gained through sensory experiences and the physical manipulation of objects. Object permanence is eventually attained in this stage. This is the ability to recognize that an object is still present even if it is unseen.

The preoperational stage, taking place between the ages of two and seven years, is the second stage of development. Children think egocentrically; it is difficult for them to see from another's perspective. While language and thinking skills are improving, children continue to think in concrete terms.

The third stage is the concrete operational stage, taking place from seven to eleven years of age. Egocentrism begins to fade and more logical thinking begins. However, this logical thinking still primarily applies to concrete objects. Children accomplish conservation, which means they understand that changing the form of an object does not change its amount or mass.

The fourth stage, the formal operational stage, begins at age twelve and progresses through adulthood. Abstract reasoning is an important characteristic of this stage. Hypothetical problems can be considered, and logical thought and problem-solving continues to improve.

Jean Piaget's theory of cognitive development was based on the concept that children are not small adults.  They have specific stages of development.  The stages included the sensorimotor stage, which occurs around the ages of 0-2. During this stage children begin to experience spatial and sensory learning.

The next stage is the Preoperational stage.  This stage generally occurs from the ages of 2-7. This stage includes the child’s abilities to begin classification.  This lets the child combine like objects in basic groups based on things like size and color. Serialization also occurs during this stage and is the ability to organize things by progression, like size, numerical values or color shadings.

The third stage is known as the concrete operational stage, ages 7-11.  In this stage the child begins to think in a more logical manner about objects and things going on around him.  The child begins to achieve success in using the concepts of numbers, "understanding of quantity, length or numbers associated with an object or process."

The fourth and final stage Paget explains is the formal operational stage. This stage begins around the age of 11 and continues to adulthood.  The abilities of the child continue to increase and expand.  The child can think more logically and begins to comprehend hypothetical ideas, the concept of the future, and becomes less literal in his/her concepts of life.

"Piaget wrote that there are four major stages of cognitive development. These stages are invariant, hierarchical, and seen in children universally."