Final answer:
Collaborative learning is the teaching method used in Piaget's stage of cognitive development for individuals aged 12-15 years.
Step-by-step explanation:
The teaching method commonly used in Piaget's stage of cognitive development for individuals aged 12-15 years is collaborative learning. In this stage, children are capable of thinking both logically about concrete events and abstractly about hypothetical situations. Collaborative learning allows students to work together in groups, sharing ideas, discussing concepts, and solving problems collectively.