In demographic studies, what typically characterizes an 'ageing population'?

An 'ageing population' is typically characterized by an increase in the proportion of old dependents. This means that there is a growing segment of the population that is aged 65 and older relative to the working-age population. As a society ages, there are implications for economies, healthcare, and social services since a larger proportion of the population will require support and care.

While a decrease in birth rates can contribute to an ageing population, it is not a defining characteristic in itself; rather, it is one of the factors that can lead to this demographic shift. Higher life expectancy among younger generations may be a result of an ageing population but does not directly characterize it. Similarly, increased immigration of young people may alleviate some of the effects of an ageing population but is not a defining trait of the population itself. Therefore, the increase in the proportion of old dependents is the most precise identifying feature of an ageing population.