The decline in fertility rates is solely responsible for the aging population, as it directly reduces the number of younger individuals in society.

The decline in fertility rates means that fewer babies are being born each year. When there are fewer young people in a population, it leads to a higher percentage of older individuals, which causes the overall population to age. For example, if a country used to have two children per family but now averages one child, there will eventually be more elderly people compared to children. This shift can create challenges, such as needing more healthcare for older people and fewer workers to support the economy. Therefore, while declining fertility rates are a major factor in an aging population, they are part of a larger picture that includes life expectancy and migration patterns.