Glossary term
Age structure
The distribution of a population across age groups — typically the proportions aged 0–14 (young), 15–64 (working-age), and 65+ (elderly). Age structure is shaped by decades of fertility, mortality, and migration history, and in turn shapes a country's dependency burden, labour supply, pension obligations, and future population trajectory.
The three canonical shapes
Age structure is visualised in a population pyramid — a back-to-back bar chart with males on the left, females on the right, and age increasing from bottom to top. Three canonical shapes correspond to stages of the demographic transition:
Expansive (wide-base pyramid): Many children, few elderly. High fertility and high (but falling) mortality. The classic shape for high-fertility developing countries like Niger, Mali, and Uganda today. The UN WPP 2024 visualises these for every country.
Stationary (rectangular): Roughly equal numbers in each age group below 65, tapering only at the top. Characteristic of countries near replacement fertility with stable mortality. Sweden and France approximate this shape.
Constrictive (top-heavy): Narrow base (few children), broad middle and top (many working-age and elderly adults). Characteristic of low-fertility, ageing countries like Japan, Germany, and South Korea. Japan's base has been narrowing for 40 years.
Age structure and economic consequences
Age structure is the mechanism through which demographic change translates into economic outcomes. The dependency ratio — dependents per 100 working-age adults — is read directly from the age structure. A country where 65% of people are working-age (15–64) has a much more favourable support ratio than one where only 55% are working-age.
This is why age structure is central to the demographic dividend: the dividend opens when falling fertility shifts the age structure toward a larger working-age share and fewer children. The World Bank estimates this structural shift accounts for 25–40% of East Asia's extraordinary growth from 1965 to 1990. The dividend closes — and becomes a burden — when the large cohort ages into retirement and the 65+ share swells.
Population momentum — age structure's legacy effect
One of the most counter-intuitive demographic phenomena is population momentum: a country's population can continue growing for decades after its TFR falls to replacement level, because of the age structure it inherited. If many women are currently in their 20s and 30s (because of high fertility 20–30 years earlier), births will be numerous even at 2.1 children per woman.
India illustrates this powerfully. Its TFR has fallen to approximately 2.0 — near replacement — yet its population continues to grow by about 10–12 million per year, because the large cohort born in the high-fertility 1990s is now at peak childbearing age. The full analysis is in our article on India surpassing China. Sub-Saharan Africa will experience even more extreme momentum — its young age structure means population growth will continue for many decades even as fertility falls.
Frequently asked questions
What is age structure?
Age structure is the distribution of a population across age groups — typically summarised as 0–14 (young), 15–64 (working-age), and 65+ (elderly) — and visualised in a population pyramid. It is shaped by decades of births, deaths, and migration, and determines dependency ratios, pension obligations, and demographic momentum.
What are the different types of age structure?
Three broad types correspond to stages of the demographic transition: (1) Expansive — wide base, many children, high-fertility countries like Niger. (2) Stationary — roughly rectangular, near replacement fertility, like Sweden. (3) Constrictive — narrow base, broad top, ageing low-fertility countries like Japan or Germany.
Why does age structure matter for the economy?
Age structure determines the dependency ratio. A large working-age share creates the demographic dividend; a large elderly share creates pension strain and labour shortages. The World Bank estimates demographic structure accounts for 25–40% of the East Asian economic miracle.
How does age structure relate to population momentum?
Population momentum means a young age structure (many people at childbearing age) causes population to keep growing even after TFR reaches replacement level. India's population continues growing by ~10–12M per year despite TFR near 2.0, because the large cohort born in the high-fertility 1990s is now at peak childbearing age.