Final answer:
Piaget's claim that cognitive development is concluded by the end of adolescence is false. His theory cites formal operational thought as the last stage, typically developed by age 20, but later studies propose continued cognitive development through a postformal stage in adulthood.
Step-by-step explanation:
The statement that Jean Piaget thought development was completed by the end of adolescence is false. Piaget is well-known for his theory of cognitive development, which includes four stages, with the final stage being formal operational thought. This stage, which revolves around abstract and hypothetical thinking, generally develops between 11 and 20 years of age. However, postformal thought is suggested by other developmental psychologists to succeed Piaget's formal operational stage, indicating cognitive development can continue into adulthood.
Furthermore, while Piaget's model emphasized distinct cognitive stages, contemporary research often supports a more continuous model of development. Improvements in processing speed and efficiency during adolescence lead to the emergence of more complex cognitive functions. Piaget's theories laid the groundwork for our understanding of cognitive development through his focus on children's cognition and how it evolves through interaction with social environments.