# Why has Australia’s centenarian population grown so quickly?

AUSTRALIAN DEMOGRAPHY

One of the features of Australia’s changing demography which doesn’t receive a lot of attention is the phenomenal growth of the centenarian population, defined as those aged 100 and above. The graph below illustrates the growth of this population from 1971 to 2021. In 1971 the number of centenarians is estimated to have been about 200.

By 2001 this had grown to almost 1,600, and by 2021 it had reached about 5,300. Over the two decades to 2021, the centenarian population grew by 235%. For comparison, the Australian population as a whole grew by 33% between 2001 and 2021. Most centenarians are female because of greater survival to advanced ages for females than males.

The centenarian population numbers shown here are not actually from the Australian Bureau of Statistics (ABS). Because of errors in reporting age at very high ages in the census, census-based population estimates in the nonagenarian (ages 90-99) and centenarian ages have a tendency to be too high. This is a common phenomenon across the world where population estimates are based on census data, and not a specifically Australian issue. More accurate estimates at high ages can be calculated indirectly from deaths data using methods described in detail __here__. Fortunately, the ABS recently adopted these methods and applied them in their __calculation__ of the centenarian Estimated Resident Population (ERP) for 2021. However, the methods have not been applied by the ABS to earlier years, so the estimates shown in the graph are my own derived via deaths data.

What is behind the rapid of growth of the centenarian population? A simple __decomposition__ can be undertaken to reveal the extent to which it is due to:

(1) increases in the number of births a century earlier,

(2) improvements in survival from birth to the centenarian ages, and

(3) increases in net overseas migration.

Let’s look at the increase in population between 2001 and 2021. Because the centenarian population, in theory, is open-ended, it’s easier to calculate if we examine the population aged 100-104, which represents about 95% of all centenarians. In 2021 this population was 3.41 times its size in 2001. The graph below shows the contribution of the 3 factors to the growth of the population aged 100-104 (persons) and for males and females separately.

The decomposition reveals that a dramatic improvement in survival is the dominant influence in the growth of the centenarian population aged 100-104 between 2001 (the cohort born 1896-1901) and 2021 (the cohort born 1916-1921). For the 1896-1901 cohort, the proportion of people surviving from birth to their 100th birthday was 0.5%, while for the 1916-1921 cohort it had increased to 1.3%.

If we consider survival from birth to ages 100-104, the factor increase is 2.422, meaning that because of improved survival the population aged 100-104 in 2021 was about 2.4 times what it would have been if survival hadn’t improved between the two cohorts.

The number of births in 1916-1921 was 28.5% greater than in 1896-1901 (giving a factor of 1.285) while net overseas migration increases only contributed a small amount to growth (a factor of 1.096).

Multiplying these three factors together gives the ratio of the size of the 100-104 age group population in 2021 to its size in 2001 (1.285 × 2.422 × 1.096 = 3.411). As the graph shows, the main difference between males and females is the greater improvement in survival for males.

In the future, continued increases in births and survival are likely to generate huge growth of the centenarian population. If long-run trends in improving survival at older ages continue, the centenarian population of Australia could reach 30,000 by 2051.