Younger children want to do what older children do.
One sunny morning as I sat near the school's playground I watched two 10-year-old girls easily and nonchalantly perform the trick of walking upright down the slide. A 6-year-old girl nearby watched them more intently than I, and then she climbed the ladder and started gingerly walking down the slide herself. This was clearly a challenge for the little girl. She walked with knees bent and hands down, ready to grab the rails if she lost balance. I also noticed that the two older girls remained next to the slide and looked on with a degree of apprehension, ready to catch her, but not too obviously so, if she should fall. One said, "You don't have to do it, you can just slide," but the little girl continued, slowly, and beamed with pride when she made it to the bottom. Shortly after that, the two older girls began climbing a nearby tree, and the younger girl followed them in that activity too. The little girl was clearly motivated to do, with effort, what the older girls could do with ease.
This is just one of many, many observations of young children modeling their behavior after that of older children. Children on the verge of being able to play strategy games, or read, or perform new operations on the computer, or engage in more advanced athletic activities, become motivated to do so by observing those activities in older children and adolescents. In our study of how and why children learn to read at the school, some told us that they wanted to read because they were envious of the older kids who were reading and talking about what they had read. As one student put it, "I wanted the same magic they had; I wanted to join that club."
Younger children don't just blindly mimic older ones. Rather, they watch, think about what they see, and incorporate what they learn into their own behavior in ways that make sense to them. Because of this, even the mistakes and unhealthy behaviors of older children can provide positive lessons for younger ones. Young children talk endlessly about what they like and don't like about the activities of the older ones around them. Negative models can be as helpful as positive ones. "I'm not going to do what X does, because I can see all the trouble it brings him."
Children also learn an enormous amount just by listening to or overhearing older ones, even when they aren't interacting with them. Through hearing the language and thoughts of older children--which are more sophisticated than their own, but not so much more so as to be out of reach--they expand their own vocabularies and range of thought.
Older children are also inspired by younger ones.
It is not just the younger children whose horizons are expanded by the age-mixed environment. At Sudbury Valley, older children and teenagers are inspired by the playthings and actions of younger ones to continue to engage in activities that they probably would have dropped by middle childhood in an age-segregated environment. They continue, for example, to play with blocks, clay, crayons, and paint. As a result, many of them become extraordinarily good at those activities. The school has produced a remarkable number of successful creative artists, and I suspect that the age-mixed environment has much to do with that.
Older children are excellent helpers and advisors of younger children, partly because they do not help or advise too much.
Children often prefer to ask an older child rather than an adult for help or advice, even when an adult is available whom they could easily ask. I suspect there are many reasons for this, but one of the main reasons, I think, has to do with control.
Children seeking help or advice do not want to give up their own control of the situation. They don't want any more help than what they ask for, and they want to decide themselves whether or not to accept what is offered. Because adults are more likely to be seen as authority figures than are older children, it is harder to reject an adult's help or walk away when advice goes beyond what the child wants. Moreover, in my observations, older children are much less likely than are adults to extend help or advice beyond what the young child wants. Older children are not worried about the long-term development of the child who has asked them for help, or about whether or not they are coming across as wonderful teachers and guides, so they just give the help that is asked for, which is all that the younger child wants.
In one of Jay Feldman's observations, for example, 5-year-old Sue asked 8-year-old Anne to thread the needle on a beading loom for her, which she needed to do to complete a bracelet she was making. After Anne threaded the needle, Sue continued her work on her own, without further help, and Anne offered none, even though Sue continued to have difficulties with the loom and made many mistakes. If Sue had asked an adult to thread the needle, rather than an older child, the adult might have hovered around and helped Sue with other parts of her project, which would have taken away Sue's pride in doing the work herself. Sue clearly didn't want such further help, even though the project was difficult for her, so it was safer to ask an 8-year-old. [Note: Students' names in this and other examples are pseudonyms.]
So, here is a valuable lesson that we adults can learn from children about helping and advising children: Don't give more help, or more advice, than is asked for! Come to think of it, the same lesson applies to helping and advising adults. I know that when I ask for help I am not asking for supervision. I just want the help I asked for. I want to do the rest myself, even if I'll make more mistakes that way. A too-helpful helper takes away my sense of freedom, self-control, and play.
Older children are excellent teachers of younger ones, partly because they are not too far ahead of the younger ones.
Daniel Greenberg made this point in one of his books about Sudbury Valley, where he wrote: "Kids love to learn from other kids. First of all, it's often easier. The child teacher is closer than an adult to the student's difficulties, having gone through them somewhat more recently. The explanations are usually simpler, better. There's less pressure, less judgment." 
Not only are the explanations simpler, but, because they come from someone closer in age, they are easier to challenge. They are more likely to be viewed as ideas to think about, rather than as Truth, and understanding comes from thought, not from blind acceptance. Here is an example from one of Jay Feldman's observations:
Eight-year-old Ed was complaining to 14-year-old Arthur about how two other boys had been teasing him by calling him names he didn't like. Arthur told Ed that he should bring a complaint to the school's Judicial Committee. Ed then said, "They have freedom of speech." Arthur, after a little thought, replied that freedom of speech meant that they had the right to say those things, but Ed also had the right not to hear them. Ed, after a little thought, said, "Okay." 
Notice that in this example Ed felt equal enough to Arthur to challenge his suggestion, and the challenge led to a new idea. Notice also the elegant language of the exchange. Big ideas were expressed in few and simple words.
Older children expand their own understanding through explanations to younger children.
Everyone who has ever been a teacher knows that we learn more when we teach than when we are taught. The requirement to put ideas into words that others can understand, and the need to think through objections that others might make, leads us to think deeply about what we thought we knew. Often this leads us to a better understanding than we had before. In an age-mixed environment, children, not just adults, can learn through teaching.
In the above example, 14-year-old Arthur, the "teacher," probably learned at least as much as 8-year-old Ed, the protégé, in their conversation. Ed's challenge to Arthur's suggestion led Arthur to think further and expand on his explanation in a way that he may not have thought about before. Both parties probably left the conversation with a deeper understanding of democracy at the school than they had before.
As another example, consider the case of an older child playing chess or some other strategy game with a younger one and teaching strategy as they play. When the older child says to the younger one that move A would be better than move B, the younger one says, "Why?" To answer this, the experienced player cannot just rely on gut instinct developed from long experience with chess, but must articulate a reason. She must turn her implicit chess knowledge into conscious, explicit knowledge, and in doing so she becomes a better chess player. Similar examples occur in every realm of exchange of knowledge and ideas among people who feel free to ask questions.
Older children develop compassion and nurturing skills through helping younger ones.
Even more valuable than the cognitive gains derived from interacting with younger children are the moral gains. To develop effectively as responsible, ethical beings, children need to have the experience of caring for others, not just the experience of being cared for by others. Observations in many cultures have shown that both boys and girls behave in more caring ways toward children who are several years younger than themselves than toward children near their own age. Little children seem to draw out the nurturing instincts that lie latent in all of us. One study, in Kenya, revealed that boys who cared for younger siblings at home behaved less aggressively, more kindly, toward same-age peers than did boys who lacked that opportunity. Apparently, the nurturing instinct is strengthened through interactions with younger children, and, once strengthened, it generalizes to age-mates.
In observations at Sudbury Valley, many examples of children nurturing younger ones can be seen every day. These include scenes of older children reading to younger ones, who sit on their laps; older children helping younger ones find lost objects or fixing things they have broken; and older children giving needed boosts to younger ones as they go about their daily activities. Some of the most interesting scenes are those in which an older child criticizes a younger one for his or her poor treatment of a still younger child. In one case, for example, we observed a 10-year-old girl explain to three 6-to-8-year-old girls why they should let a certain 4-year-old join them in their game. "How would you feel if you weren't included," she said. In another case we observed a 17-year-old boy reprimand a 13-year-old for his unfriendly way of rejecting an 8-year-old boy who asked to play a game with him. The reprimands we heard in these examples were much more effective coming from an older child than they would have been if they had come from an adult.
Taking this essay along with the previous two, I conclude with the following summary. An age-mixed environment (1) allows younger children to engage collaboratively in activities that they could not do just with age-mates; (2) promotes non-competitive, creative forms of play that are ideal for acquiring new skills; (3) allows those who are ahead of or behind their age-mates in certain realms to find others who are at their level; (4) permits younger children to be inspired by the activities of older ones, and vice versa; (5) allows younger children to receive help and advice without giving up their own autonomy; (6) allows older children to learn through teaching; and (7) allows older children to practice caring for younger ones and to develop a sense of responsibility and maturity. When we segregate children by age, in schools and in other settings, we deprive them of all of this. We rob them of the opportunity to use fully their natural and joyful ways of learning from one another.
 Feldman, J. (1997). The educational opportunities that lie in self-directed age mixing among children and adolescents. Ph.D. dissertation, Department of Psychology, Boston College, 1997.
 Greenberg, D. (1987). Free at last: the Sudbury Valley School. Framingham, MA: SudburyValley School Press.
 Gray. P. & Feldman, J. (2004). Playing in the zone of proximal development: Qualities of self-directed age mixing between adolescents and young children at a democratic school. American Journal of Education, 110, 108-145.
 Ember, C. R. (1973).Feminine task assignment and the social behavior of boys. Ethos, 1, 424-439.