Final answer:
An adolescent engaging in hypothetical thinking and speaking of possibilities is likely in the formal operational stage of Piaget's cognitive development theory, which begins around age 11.
Step-by-step explanation:
An adolescent who hypothesizes and speaks in terms of possibilities is likely functioning in Piaget's cognitive stage of formal operational thought. This is the final stage in Piaget's theory of cognitive development, and it typically developes from age 11 and onwards. In this stage, individuals are capable of abstract thinking and can formulate hypotheses, imagine hypothetical situations, and consider various solutions to problems. They move beyond concrete thinking to engage in more complex, abstract reasoning, indicating significant cognitive maturation.