This excellent essay by Peter Gray has been making the rounds. I have nothing to add — read it!

It is sad to see, in our age-graded society, that many if not most children and adolescents have few opportunities to get to know and to interact regularly with children who are much younger than themselves. If we want young people to grow up to be compassionate and caring, we need to allow them to exercise those capacities; and to do that we need to break down the barriers we have erected to keep young people of different ages apart. We are designed by nature to learn to be compassionate by observing and caring for littler ones while we ourselves are growing up.