Final answer:
Piaget's formal operational stage is when individuals develop the ability to think logically about abstract propositions, beginning from approximately age 11 onwards.
Step-by-step explanation:
According to Piaget, the ability to think logically about abstract propositions is indicative of the formal operational stage. This stage is the fourth and final stage in Piaget's theory of cognitive development, which begins from about age 11 into adulthood. It is characterized by the capacity for abstract thinking and problem-solving, allowing individuals to consider hypothetical scenarios, engage in logical reasoning, and think about abstract relationships. In contrast, the pre-operational stage involves symbolic thinking and egocentrism, the concrete operational stage involves logical thought about concrete objects, and post-conventional thought (not one of Piaget's stages) relates to Kohlberg's theory of moral development.