The adolescent stage in Piaget's theory of cognitive development begins - typically about the age of 11 or 12 years - with the emergence of what he called “formal reasoning” or “formal operations.” Such reasoning may continue to develop over the course of adolescence and early adulthood, and some individuals may even construct more advanced forms of cognition, but the transition from childhood to adolescence marks the last major qualitative transformation highlighted in Piagetian theory and research. Thus, Piaget's theory differs, on the one hand, from neonativist and other theories that see development as fundamentally a phenomenon of early childhood, and on the other hand from lifespan theories that see development as continuing inexorably through adulthood. Piaget's theory leaves open the possibility of more advanced development, but he did not describe a stage beyond formal operations.

By “formal reasoning,” Piaget meant hypothetico-deductive reasoning. Even young children make inferences, and by age 7 or 8 their inferences conform to strict rules of deductive logic, but such inferences always begin, Piaget maintained, with what the child believes or accepts. Beginning about age 11 or 12, thinkers explore what can be deduced from propositions deemed hypothetical or even false. This is hypothetico-deductive, or formal, reasoning. Piaget's conception of the emergence of formal reasoning at the transition to adolescence can already be seen in his early work from the 1920s.