Final answer:
False, Piaget did not propose a fifth stage of cognitive development; the idea of a postformal stage was suggested by other developmental psychologists and is not part of Piaget's original four stages.
Step-by-step explanation:
The statement Piaget proposed a fifth stage of cognitive development, the postformal stage, is false. Jean Piaget, a renowned developmental psychologist, created a comprehensive theory of cognitive development in children which includes four distinct stages: the sensorimotor stage, the preoperational stage, the concrete operational stage, and the formal operational stage. The final stage, the formal operational stage, is characterized by the ability to think abstractly and hypothetically. It was believed by Piaget to begin at around age 11 and extend into adulthood. However, it has been other developmental psychologists who suggested the existence of a postformal stage of cognitive development, which is not officially recognized by Piaget's theory. In this proposed postformal stage, thinking becomes more contextual, flexible, and integrated with emotion.