From an evolutionary perspective, the normal social play of children involves kids of various ages. Our human and great-ape ancestors most likely lived in small groups with low birth rates, which made play with others of nearly the same age rare. Consequently, the evolutionary functions of children's social play are best understood by examining play in groups that include children of different ages. The author calls this kind of play "age mixed." He reviews the research on such play, including his own research conducted at the Sudbury Valley School in Massachusetts where students from ages four to about eighteen mix freely. He concludes that age-mixed play offers opportunities for learning and development not present in play among those close in age, permitting younger children to learn more from older playmates than they could from playing with only their peers. In age-mixed play, the more sophisticated behavior of older children offers role models for younger children, who also typically receive more emotional support from older kids than from those near their own age. Age-mixed play also permits older children to learn by teaching and to practice nurturance and leadership; and they are often inspired by the imagination and creativity of their younger playmates. (Contains 45 notes.)