The authors argue that the basic length of both generations and turnings—about twenty years—derives from longstanding socially and biologically determined phases of life.^[who?] This is the reason it has remained relatively constant over centuries.^[80] Some have argued that rapid increases in technology in recent decades are shortening the length of a generation.^[81] According to Strauss and Howe, however, this is not the case. As long as the transition to adulthood occurs around age 20, the transition to midlife around age 40, and the transition to old age around age 60, they say the basic length of both generations and turnings will remain the same.^[80]

In their book, The Fourth Turning, however, Strauss and Howe say that the precise boundaries of generations and turnings are erratic. The generational rhythm is not like certain simple, inorganic cycles in physics or astronomy, where time and periodicity can be predicted to the second. Instead, it resembles the complex, organic cycles of biology, where basic intervals endure but precise timing is difficult to predict. Strauss and Howe compare the saecular rhythm to the four seasons, which they say similarly occur in the same order, but with slightly varying timing. Just as winter may come sooner or later, and be more or less severe in any given year, the same is true of a Fourth Turning in any given saeculum.^[82]