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Population aging is the increase in the proportion of older persons (typically 65+) in a population, driven primarily by fertility decline rather than mortality improvement. When fertility falls, fewer young people enter the population, shifting the age distribution upward. Mortality decline at older ages reinforces aging by extending life at the top of the pyramid. The pace and stage of aging vary enormously: Japan has the world's oldest population (29% aged 65+), while Niger has one of the youngest (2.5%). Aging creates challenges for pension systems, healthcare, labor markets, and intergenerational equity, but also brings opportunities through the experience and accumulated capital of older populations. The demographic old-age dependency ratio and related measures quantify the scale of these challenges.
Compare the demographic trajectories of Japan (aging since the 1970s) and a sub-Saharan African country (just beginning to age). Plot the share of 65+ over time for both countries and project forward. The divergent timelines — and the realization that today's youngest countries will face the same challenges within decades — clarify that aging is a universal outcome of the demographic transition.
From population pyramids, you understand how age structure reflects the accumulated history of fertility, mortality, and migration. Population aging is the long-run consequence of one particular structural shift: the decline in fertility that accompanies the demographic transition. As fewer children are born, the base of the pyramid narrows, and the proportion of the population in older age groups increases — even if those age groups have not grown in absolute numbers.
This mechanism — aging from the bottom — is counterintuitively the primary driver of population aging. Most people assume that aging is caused by people living longer, and mortality improvement does contribute (aging from the top). But consider the arithmetic: if a population has a TFR of 5, roughly 15-18% of the population is under age 5 at any time. If TFR drops to 1.5, that percentage falls to perhaps 4-5%. The proportion of people aged 65+ rises not because more old people exist but because fewer young people do. In virtually every country that has experienced substantial aging, fertility decline explains the majority of the proportional shift.
The pace of aging varies enormously and is accelerating historically. France took over 100 years for its 65+ share to double from 7% to 14%. Japan accomplished the same doubling in 24 years. China will do it in about 25 years, and many developing countries face even more compressed timelines. This creates a fundamental challenge: countries that aged slowly had generations to build pension systems, retrain workforces, and adapt healthcare infrastructure. Countries aging rapidly must accomplish the same institutional adaptation in a fraction of the time, often at lower income levels.
The economic implications center on the support ratio — the number of working-age adults per retiree. When this ratio falls (as it is doing in virtually every post-transition country), fewer workers must generate the economic output to support pension payments, healthcare costs, and long-term care for a growing elderly population. Possible responses include raising retirement ages, increasing immigration of working-age adults, raising productivity through technology, and restructuring pension systems from defined-benefit to defined-contribution. None of these is sufficient alone, and all involve intergenerational tradeoffs. The dependency ratio, which you will study next, formalizes this relationship.
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