I work with children in many different educational settings and see a wide range of socialization skills. The most mature students are the ones who have access to all ranges of ages throughout their day.
When confined to same age grouping children are given few opportunities to learn appropriate social skills. A typical student in school comes into contact several hundred same age peers and a few adults, all of which are in a role of superior to the child.
For the children not attending an organized school setting, children will come into contact with younger children, older children and adults who are equal in ranking to the student in addition to those in a superior roll. It is in these setting that I see most emotional growth.