Population Pyramids
A country's age structure determines almost everything about its future: workforce, pensions, healthcare demand, political dynamics, growth potential. Sub-Saharan Africa has classic broad-based pyramids (high fertility, many young people). Most of Europe shows rectangular structures (low fertility, broad middle). Japan and Italy show inverted patterns (more 70-year-olds than 30-year-olds).
Key insights
Three pyramid shapes
Expansive (pyramid-shaped): high birth rate, broad base, narrows quickly toward old age. Niger, Mali, DRC, Afghanistan. Constrictive (rectangular): low fertility, broad middle, narrower base. Most of OECD, China. Stationary (inverted): fertility well below replacement, more older people than younger. Japan, Italy, Germany, South Korea (extreme case).
Niger and Japan are demographic opposites
Niger: median age 14.6, total fertility rate 6.6 — over 100 children per 100 working-age adults. Japan: median age 48, TFR 1.3 — about 50 elderly per 100 working-age adults. The two countries have similar populations (~125M) but couldn't be more different in age structure. Their economic, political and policy challenges have almost no overlap.
Pyramid shape is mostly destiny
Population age structure changes slowly. A country with a baby boom in 2020 has an age-65 spike in 2085 — visible in the data 65 years in advance. Most of the demographic dividends and burdens of 2050 can already be read off today's pyramids. The major uncertainty is migration, which can reshape structure over decades.
Median age — selected countries (2024)
Years
Key Finding: 30-year gap between oldest and youngest major countries — and it's widening.
Population by age group — Niger vs Japan (2024)
% of total population in each broad band
Key Finding: Niger has 16× more children under 5 per old person than Japan. The economic implications are vast.
Methodology & caveats
Dependency ratios
Child dependency = under 15 / 15-64. Old-age dependency = 65+ / 15-64. Total dependency = (under 15 + 65+) / 15-64. Countries with expansive pyramids have high child dependency; countries with constrictive pyramids have high old-age dependency. The combined burden is U-shaped over the demographic transition.
Why median age is a useful single number
Median age summarizes the entire age structure into one statistic. It's intuitive (half are younger, half older), comparable across countries, robust to definitional choices. A country's median age tomorrow is essentially set by today's age structure plus differential mortality — projection is unusually reliable.
Migration shifts the picture
International migration is the only fast lever for age structure. The US working-age share has held up better than peer countries partly through immigration of working-age people. Germany absorbed ~1M Ukrainian refugees in 2022, most working-age. The UK relies heavily on EU- and Commonwealth migration. Most fertility-decline-driven aging cannot be reversed — only partially offset by migration.