This is a city or town that works for children and old folks. Because the old folks and the children are using the streets together, the children are learning from the old folks the principles and values that will allow them to grow old and then teach the same principles and values to the next generation of children, and so forth. When we talk of sustainability in nature we mean that the ecology of a place works in such a way that the plants and creatures of the place reproduce so that the ecological balance and health of the place is maintained and sustained. A community that sustains and maintains itself in health and comfort can only happen when a community is designed for children, and the children are enculturated by the adults and old folks that they are safely interacting with all the time. The old folks respect and watch the children, and the children venerate the adults and learn the communities' traditions and values from them.
Children can safely explore a world that is much larger and more diverse than their own rear yard.
They can play safely in the streets with other neighborhood children without constant supervision. This gives them the opportunity to experience serendipitous natural relationships with other children - not just relationships structured and managed by adults. Nature is ecologically sustainable because it is complex, diverse, interdependent, integrated, and decentrailzed. For our human culture to sustain itself, it must be founded on similar principles. Until the last 50 years, neighborhoods and communities had always incorporated these age-old natural values.