today. Because most new immigrants who arrive in the next decades will be younger than that age, immigration flows will not fundamentally alter the size of this age group. Rather, the size of the oldest-old population will depend mainly on future trends in mortality.
One useful concept in population analysis, the ratio of the number of people in their "dependent" years to those who are in their working years, is especially relevant to the study of immigration. Variations in age dependency reflect in an overall way the contribution of variations in age composition to economic dependency. Dependency ratios based on age, it should be emphasized, do not equate with economic dependency. We report three dependency ratios here: (1) a youth dependency ratio, for the number of persons 19 years of age or under per 100 persons in the working years, aged 20 to 64 years; (2) an elderly dependency ratio, relating persons aged 65 years and over to those in the working years; and (3) an overall dependency ratio relating these two groups together to those in the working years.
As can be seen in Figure 3.5, the youth dependency ratio reached a peak in the 1960s as the large number of those born in the baby boom were of school-age.