A decomposition of sources of change in population size and median age, 1970-2020

Thomas Pullum, Demographic and Health Surveys Program
Apoorva Jadhav, U.S. Agency for International Development (USAID)

The median age of a population has implications for many aspects of a country’s development, economic growth, and socio-political outcomes. Yet, there is no clear consensus on which aspects of population dynamics contribute to change in median age over time. In this paper, we examine changes in population size and median age from 1970 to 2020 in five-year time intervals, using estimates from World Population Prospects 2019 for regions of the world. Changes are described with parallel models that include six components of change within each time interval: births, deaths to those births, other deaths below the median age, deaths above the median age, and net migration below and above the median age. Population size and median age in 2020 are compared with what would have been expected in 2020 if the 1965-70 rates had continued, or if only one component at a time had followed the observed trajectory. We find that changes in fertility levels are responsible for a large part of the change in median age in many regions of the world, but world regions vary considerably in the balance among the components of change.

Keywords: Age structure, Population size and growth/decline, Population ageing, Decomposition analysis/methods

See paper.

  Presented in Session 113. Measurement of Age and Age Structures