3.6 Age Structure Diagrams

What age structure diagrams show

An age structure diagram (population pyramid) shows the number or percentage of individuals in each age group, split by sex, for a population. Rapidly growing populations have a wide base (many young), stable populations have roughly even-width bars, and shrinking populations have a narrower base than middle/top.

Predicting future growth

The shape predicts future trends: a pyramid shape signals high birth rates and future growth (common in developing countries), a column/rectangle shape signals stable population (zero population growth), and an inverted pyramid (narrow base) signals population decline, often seen in developed, aging countries like Japan.

Reading the bars

Each horizontal bar represents an age cohort (e.g., 0-4, 5-9 years), with males typically on the left and females on the right. The pre-reproductive (0-14), reproductive (15-44), and post-reproductive (45+) groups help estimate future birth and death rates and overall population momentum.

Key terms

Age structure diagram

A graph showing the number/percentage of males and females in each age group of a population.

Pyramid-shaped diagram

Wide base, narrow top — indicates high birth rates and rapid future population growth.

Column-shaped diagram

Roughly even bar widths across ages — indicates a stable population with zero growth.

Inverted pyramid diagram

Narrow base, wider top — indicates declining population, often in aging developed countries.

Pre-reproductive, reproductive, post-reproductive

Age categories (0-14, 15-44, 45+) used to estimate future births, deaths, and population trends.

Try an AP-style question

An age structure diagram has a very wide base that narrows quickly. What does this predict?

1. A stable population
2. No births
3. Population decline
4. Rapid population growth