If a country's working-age share first rises and later begins to decline, what is a likely long-term demographic cause?

Correct answer: A large working-age cohort is entering old age while younger cohorts are smaller

The working-age share depends strongly on the size of different birth cohorts as they move through the age structure. It can first rise when a large group of young people reaches working age and the younger group is relatively smaller. Later, that same large group grows older and leaves the working-age category. If fertility has fallen, the cohorts behind it will be smaller, so fewer young people enter working age to replace those leaving it.

Thus option A is the most likely long-term demographic cause. This pattern is often related to a demographic dividend followed by population ageing, although migration and mortality can also influence the exact timing. Working-age adults do not suddenly become children, and extremely high fertility would usually enlarge future young cohorts rather than explain a later decline in the working-age share. Age structure does change over time.