US Culture, Adults in Their Early 20s Are Expected to Marry

U.S. culture, adults in their early 20s are expected to marry, start families, establish themselves in their jobs or careers, and settle into separate households; 45-year-olds are expected to be launching their children into independence, to be reaching the peak of their careers, and to be caring for their own aging parents. Such expectations illustrate which concept?

A) cohorts
B) generations
C) shared experiences
D) cross-sectional comparisons