"One of the oddest, and in my view most harmful, aspects our treatment of children today is our penchant for segregating them into separate groups by age. We do that not only in schools, but increasingly in out-of-school settings as well. In doing so, we deprive children of a valuable component of their natural means of self-education." - Lit

"When given a choice, children spend considerable time interacting with others who are older or younger than themselves. Sudbury Valley has, at any given time, approximately 170 to 200 students, who range in age from 4 to 18 years old and sometimes older. Students can move freely at all times throughout the school buildings and campus, and they can interact with whomever they please. The school is large enough that students could, if they chose, interact just with others who are within a year or two of themselves in age. But they don't do that. In our quantitative study we found that more than 50% of students' social interactions at the school were with other students who were more than two years older or younger than themselves, and 25% of their interactions were with other students who were more than 4 years older or younger than themselves [2]. Age mixing was especially frequent during play. Active play of all sorts was more likely to be age mixed than was conversation that did not involve play." - Lit

"Age mixing allows younger children to engage in, and learn from, activities that they could not do alone or only with age-mates. In the 1930's, the Russian developmental psychologist Lev Vygotsky developed a concept that he called the zone of proximal development, defined as the realm of activities that a child can accomplish in collaboration with more skilled others but cannot accomplish alone or with others who are at his or her same level [4]. Vygotsky claimed that children learn best when they are engaged with more skilled others within their zones of proximal development. Since Vygotsky's time, education professors have often used Vygotsky's concept to describe interactions between adult teachers and young learners, but the concept applies far better, I think, to naturally occurring age-mixed interactions among children." - Lit