Final answer:
Children become able to deal effectively with abstract concepts during the formal operational stage in Piaget's theory of cognitive development.
Step-by-step explanation:
In Piaget's theory of cognitive development, children become able to deal effectively with abstract concepts during the formal operational stage. This stage occurs from about age 11 to adulthood. In the formal operational stage, children can use abstract thinking to problem solve, look at alternative solutions, and test these. They can also deal with abstract ideas and hypothetical situations. For example, they can understand and solve algebraic equations or consider complex moral dilemmas.