The Special Value of Children's Age-Mixed Play by Peter Gray (pdf)
When I first saw this article in the American Journal of Play, I thought it was brilliant. So glad to see it is available online!
I have a child who will be entering a Sudbury school next year. Tomorrow she starts her visiting week, in fact. It will be my first experience with the method, beyond talking with Barbara Dewey's son last summer (who is a Sudbury teacher). It was he who first turned me on to the possibility. But I thought the above article on mixed age play was excellent and that topic is best studied at Sudbury schools because of their unique model.