Referenced by 26 other explainers
Age is a core demographic variable. It is one of three axes used in the cohort-component method. The other two are sex and year. Every person in a forecast sits in one of these age by sex by year cells.
Age tells us how long a person has lived. It shapes the risk of most demographic events. The young have high migration rates. Older people have high mortality rates. Females in their twenties and thirties have the highest fertility rates. For these reasons, age is the backbone of any population model.
Place Forecast uses age last birthday. This is the standard ABS convention. A person who has lived 27 full years and is partway through their 28th year has an age of 27. Age is stored as a whole number.
Place Forecast uses single year of age (SYOA) from 0 to 85+. Each year of age is its own bin. This gives fine detail and matches how the ABS publishes its data.
Place Forecast records age at 30 June each year. This matches the Estimated Resident Population, which is a stock at 30 June. Using one fixed date keeps the data clean.
A person born on 1 July 1990 is aged 34 at 30 June 2025. A person born on 30 June 1990 is aged 35 at that same date. This single cut-off makes rates and flows line up between years.
The top age group is open-ended. It holds everyone aged 85 or more. Place Forecast uses this group because counts thin out at older ages. Having a bin for each single year above 85 would give noisy, small counts.
The 85+ group behaves in a special way. People age into it each year but do not age out. Only death or out-migration can remove them.
Most demographic rates vary by age. A rate is events divided by a matched population at risk. Place Forecast computes a separate rate for each age and sex. See the occurrence rate and population at risk pages for the full method.
A cohort is a group of people born in the same year. At any point in time, a cohort has one age. A cohort born in 2000 is aged 25 at 30 June 2025. The next year, the same cohort is aged 26.
Age groups and cohorts move together through the projection. This is why the cohort-component method is named as it is.
Age reporting is strong in Australia but not perfect. Some people round their age to the nearest five or ten. This is called age heaping. The ABS smooths the Census counts to reduce this effect. Place Forecast uses the smoothed data and does not apply further smoothing.
Place Forecast stores age as single year of age from 0 to 85+. Age is always age at 30 June. Age-specific fertility rates apply to females of child-bearing age. Mortality rates by age and sex come from life tables. The births age profile, deaths age profile, in-migrations age profile, and out-migrations age profile all use age as their key. Age also drives the pyramid display and the ageing report.