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How has declining human mortality changed the age structure of the world population?

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Line graph showing world population growth from 1700 to 2000 with a steep rise after 1950, followed by three projection scenarios for 2000 to 2100: high, medium, and low, reaching between 7 and 14 billion. — population Graph of the world's estimated human population from 1700 until 2000, with population projections extending until 2100.

While it is a common belief that mortality rates are the primary driver of changes in age structure, demographers emphasize that fertility rates have a more significant impact on the overall shape of a population pyramid. Because mortality declines generally affect all age groups, their influence on the proportion of people in specific age categories is relatively limited compared to fertility changes, which specifically alter the number of newborns.1

However, declining mortality—particularly the epidemiologic transition from high childhood mortality to a pattern where degenerative diseases affect the elderly—has fundamentally shifted population structures. As survival rates have increased, especially in industrialized nations, the proportion of older individuals has grown. This has led to:234

Increased Longevity: Significant improvements in life expectancy have resulted in larger numbers of people surviving past age 65.45
Growth of the Very Old: Declines in mortality among the elderly have caused the "frail elderly" population (aged 85 and older) to increase rapidly, creating new challenges for health care and social systems.64
Demographic Transition: In later stages of demographic transition, reduced childhood mortality combined with lower fertility leads to a more stable population with a broader peak in the age structure.3