 Two children in school who originally can solve problems at an eight-year-old developmental level (that is, typical for children who were age 8), might be at different developmental levels.If each child received assistance from an adult, one was able to perform at a nine-year-old level and one was able to perform at a twelve-year-old level.He said “This difference between twelve and eight, or between nine and eight, is what we call "the zone of proximal development.