Demographic Transition and Population Ageing

Demographic transition is the shift from high birth and death rates to low birth and death rates as societies develop. Population ageing means an increase in older people's share of the total population, not merely an increase in their absolute number. Advanced industrial economies age because sustained low fertility reduces younger cohorts while longer survival increases the numbers reaching older ages.

The transition from high to low birth and death rates

Demographic transition changes both population growth and age composition. Mortality usually declines before fertility, so ageing is not an immediate consequence of the transition.

• Initially, high birth and death rates produce relatively slow population growth, while high fertility sustains a youthful age structure.
• Improvements in sanitation, nutrition and healthcare lower mortality; when birth rates remain high, population growth accelerates.
• With urbanisation, education and access to contraception, fertility declines, and population growth slows as birth and death rates become low.

How the age structure shifts towards older ages

Declining fertility is a central driver of population ageing because it reduces the size of successive birth cohorts relative to older generations. Longer survival, especially at adult and older ages, reinforces this shift.

• Fewer births reduce the proportion of children, so older people can form a larger population share even without an immediate rise in their absolute number.
• As larger cohorts born before fertility declined move through adulthood into old age, they are followed by smaller younger cohorts.
• Lower mortality at older ages allows more people to survive longer after reaching retirement ages.
• Early reductions in infant and child mortality can initially strengthen younger cohorts; mortality decline does not automatically produce ageing at every stage.

Why ageing persists in advanced industrial economies

Many advanced economies combine sustained low fertility with high life expectancy. Where fertility remains below replacement level, younger generations become progressively smaller in the absence of sufficient offsetting migration, reinforcing ageing over time.

• Age structure changes slowly because existing cohorts continue ageing; a fertility increase cannot immediately expand the working-age population.
• The old-age dependency ratio, which compares older people with the working-age population, tends to rise, creating pressures on pensions, healthcare and care services.
• Working-age immigration can moderate ageing and help meet labour demand, but migrants also age, so immigration does not permanently eliminate the underlying process.
• The economic burden depends also on employment, productivity and retirement patterns, so an older population does not mechanically imply economic decline.