Biologically and socially, a full human life is divided into four phases: childhood, young adulthood, midlife, and elderhood. Each phase of life is the same length as the others, capable of holding one generation at a time. And each phase is associated with a specific social role that conditions how its occupants perceive the world and act on those perceptions.
The Fourth Turning by William Strauss and Neil Howe (pg. 16)
