Why fertility, population growth and median age differ

Separate size from rate

Total population describes size; population growth describes change; fertility describes a period's childbearing pattern. Their units differ: people, percent per year and births per woman.

Low fertility and positive population growth can coexist. Instead of assuming an error, distinguish population composition, births, deaths and migration. A single observation cannot determine a cause or future path.

Median age and life expectancy differ too

Median age describes the middle of the age distribution; life expectancy summarizes mortality conditions. Subtracting median age from life expectancy does not give residents' remaining lifespan, and median age does not give the older-age share.

Hypothetical example: ages 10, 20, 30, 40 and 90 have a median of 30 and a mean of 38. Different summaries of the same group can differ. These are teaching numbers, not country data.

Read the page in context

On Country pages, check each card's year and source. Compare uses common periods within an indicator, not necessarily across indicators. Multiplying a WDI population by a WPP rate does not produce an official population-change count.

An unavailable trend is not filled with a guessed line. Missing historical display does not mean zero or an unchanged population. Use trends only where sufficient publishable observations are available.

Evidence and official references

These examples read the published common-period values when this page is opened. Editorial dates are not data reference periods; values can change with source revisions.

Check the published data

The notes below use the currently published values. Years and sources may differ across indicators.

Median age

Japan 49 years / South Korea 44.5 years

Reference period: 2023 · UN DESA Population Division · World Population Prospects 2024

Estimated

Raw difference, left minus right: 4.47 years. The raw value for Japan is higher than for South Korea. This establishes neither superiority nor the cause of the difference. Median age differs from mean age. It locates the middle of the population, but does not directly give the older-age or working-age share.

Total fertility rate

Japan 1.208 births per woman / South Korea 0.72 births per woman