The quiescent phase in human mortality: when do populations start to age?

Michal Engelman, University of Chicago
Ravi Varadhan, Johns Hopkins University
Christopher L. Seplaki, University of Rochester

Demographic studies of mortality hazards tend to emphasize the two ends of the lifespan, focusing on the declining pattern of child mortality or the increasing trajectory of deaths at older ages. We call attention to the relatively quiescent phase in between, representing the ages when we are least vulnerable to the force of mortality. Using nonlinear fitting methods and data for female and male cohorts born 1800-1919, we estimate the parameters of the Siler model – which describes the mortality hazard across the full age spectrum. We then calculate inflection points in the Siler curve and characterize the start of the quiescent phase (representing the end of childhood vulnerability) and its end (the age when the hazard begins its exponential rise). As mortality hazards declined over time, the quiescent phase has gotten longer, with an earlier beginning in early childhood and a later end at the conclusion of the reproductive years. This analysis provides new insight into the dynamics of human development and implications for studies of individual and population aging.