Free to Learn, Peter Gray provides a strong indictment against our age-segregated systems of formal schooling.
Children learn by observing and interacting with other(s) (children) who are older and younger than they are....the segregation of children by age is an oddity -- I would say a tragic oddity -- of modern times.
Not until the large-scale expansion of compulsory, age-graded schooling, beginning about a hundred years ago in Western societies, were large numbers of children required to spend significant amounts of time in age-segregated settings.
Within the past three or four decades, in the United States and many other Westernized nations, the degree of age segregation imposed on children has increased further, to a startling degree.
The decline in the size of nuclear families, the weakening of extended family ties, fears about negative influences that older children might have on younger ones, the decline in free neighborhood play, the increased amounts of time spent at school, and the proliferation of after-school programs and other adult-directed, age-segregated activities for children have conspired to reduce greatly children's opportunities to get to know others who are several years older or younger than them. The graded school model has commandeered our culture's thought about childhood. p. 182-184