8.1.10 (HL)—Age-sex pyramids

Dependency ratio compares dependent-age groups (commonly ages 0–14 and 65+) with the productive-age population aged 15–64; population momentum is continued growth caused by a large young cohort even after fertility falls.

Calculate (population aged 0–14 + population aged 65+) ÷ population aged 15–64 × 100. Momentum occurs because many people are already approaching reproductive ages, so today's age structure carries growth forward.

If 300 young and older dependents rely on 600 people aged 15–64, the dependency ratio is 50 dependents per 100 productive-age people; a large young cohort can still raise total births after fertility per woman drops.

Falling fertility does not necessarily stop growth immediately because cohort size and the number entering reproductive ages also matter.

The standard age bands are a model: schooling, retirement and employment vary among societies.