Final answer:
Children reach the formal operational stage in Piaget's theory of cognitive development when they are capable of abstract thinking, starting at about age 11.
Step-by-step explanation:
According to Piaget, when children are capable of abstract thinking, they have reached the level of development called the formal operational stage. This stage is the fourth and final in Piaget's theory of cognitive development and begins around age 11. It stands in contrast to the concrete operational stage, which occurs from about 7 to 11 years old, where children think logically only about real, tangible events.
In the formal operational stage, adolescents can theorize, use logical reasoning to solve abstract problems, and think about hypothetical situations. They can deal with abstract ideas, which allows them to problem solve in a more advanced way, looking at different potential solutions and testing these hypotheses.