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Why can a larger proportion of older people contribute to slower population growth?

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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.

A larger proportion of older people contributes to slower population growth primarily by altering the age structure of a population. When a significant segment of the population is aged 65 or older, there are proportionately fewer individuals within the reproductive age range. Because fertility is constrained by the number of individuals capable of reproducing, this shift reduces the overall number of births in a given period.1

Furthermore, this phenomenon is often linked to the process of population aging, which occurs when low fertility rates are combined with increased life expectancy. As the proportion of older people increases relative to the working-age population, the demographic profile shifts away from the high-growth patterns seen in more youthful populations. In biological and ecological terms, while some species may employ different life history strategies to maximize reproductive output, human populations experiencing these demographic shifts typically see a decline in the total number of offspring produced, leading to a slower or negative rate of population growth.2314