Turnings occur every 20-25 years or so, resulting, ultimately in the Fourth Turning that completes a human life cycle of a generation. According to Wikipedia, the four 20-25 year turnings are part of a larger cyclical "saeculum" (a long human life, which usually spans between 80 and 100 years, although some saecula have lasted longer). A crisis recurs in American history after every saeculum, so every 80 years or so, which is happening to us now - in the 2000s.
The Fourth Turning is an era of destruction, often involving war or revolution, in which institutional life is destroyed and rebuilt in response to a perceived threat to the nation's survival. After the crisis, civic authority revives, cultural expression redirects towards community purpose, and people begin to locate themselves as members of a larger group. The previous Fourth Turning in the US began with the Wall Street Crash of 1929 and climaxed with the end of World War II.
The G.I. Generation (which they call a Hero archetype, born 1901 to 1924) came of age during this era. They say their confidence, optimism, and collective outlook epitomized the mood of that era. The Millennial Generation (which they also describe as a Hero archetype, born 1982 to 2004) show many similar traits to those of the G.I. youth, which they describe as including: rising civic engagement, improving behavior, and collective confidence.