Piaget’s stages of cognitive development:
1. Sensorimotor stage (Infancy). In this period (which has 6 stages), intelligence is demonstrated through motor activity without the use of symbols. Knowledge of the world is limited (but developing) because its based on physical interactions/experiences. Children acquire object permanence at about 7 months of age (memory). Physical development (mobility) allows the child to begin developing new intellectual abilities. Some symbollic (language) abilities are developed at the end of this stage.
2. Pre-operational stage (Toddler and Early Childhood). In this period (which has two substages), intelligence is demonstrated through the use of symbols, language use matures, and memory and imagination are developed, but thinking is done in a nonlogical, nonreversable manner. Egocentric thinking predominates.
3. Concrete operational stage (Elementary and early adolescence). In this stage (characterized by 7 types of conservation: number, length, liquid, mass, weight, area, volume), intelligence is demonstarted through logical and systematic manipulation of symbols related to concrete objects. Operational thinking develops (mental actions that are reversible). Egocentric thought diminishes.
4. Formal operational stage (Adolescence and adulthood). In this stage, intelligence is demonstrated through the logical use of symbols related to abstract concepts. Early in the period there is a return to egocentric thought. Only 35% of high school graduates in industrialized countries obtain formal operations; many people do not think formally during adulthood.
Huitt, W., & Hummel, J. (2003). “Piaget’s theory of cognitive development.” Educational Psychology Interactive. Valdosta, GA: Valdosta State University. Retrieved 6-21-09 from http://chiron.valdosta.edu/whuitt/col/cogsys/piaget.html
This is what I have (generally, non-scientifically) observed, too — but I had not put it to words, had not done a lot of work and research in the area, had not fully developed a theory, had not put it all into a science as Piaget had. Thanks to Piaget!