Ageing badly: indicators of old-age structure in Australia and New Zealand

KEYwords: Indicators of old-age structure; Māori; Aboriginal; centenarians

Two decades ago, a description of the characteristics of New Zealand’s centenarians was published, with the authors concluding that extreme ageing can be used as a basis for comparison with other countries.¹ No single indicator is good enough to assess longevity. To obtain a more reliable estimate of longevity and population ageing, epidemiological indicators, in addition to those reflecting extreme ageing, are needed.² The ratio between ultra-non-agenarians (people aged 90 – 99 years, inclusive) and the total population aged >65 years (longevity index: LI %) and the ratio of centenarians to the total population aged >90 years (centenarity index: CeI %) are two proposed indices. Increases in life expectancy have not been uniform across ethnic and socioeconomic groups. The older a person becomes, the more extreme is the longevity phenotype. At 90 years of age, one is close to the 90th percentile of the life expectancy and at age 100 years, people are beyond the 99th percentile for contemporary birth cohorts. Thus, the use of multiple indicators such as LI or CeI better represents healthy longevity.³

The presence of people older than 65, 90 and 100 years in the total population of various countries may be used in calculating various indices that reflect the quality of ageing on a national level better than simply using mean life-expectancy or percentage of older adults in a country. Population ageing measured through a fixed old-age threshold (usually ≥60 years or ≥65 years of age) ignores other key dimensions of ageing such as years lived with disability.⁴ Understanding population ageing across countries with distinct demographic and health achievements calls for a combination of ageing indices.⁵ Many important characteristics of people diverge with age, but age-specific characteristics also vary over time and differ from place to place. Augmenting the measurement of chronological age with indices that take into account the changing characteristics of populations allows more broad and precise analysis of ageing.⁶

Between 2013 and 2050, Australia and New Zealand are expected to experience an increase in the proportion of people aged ≥60 years. Increases in the population aged ≥80 years will be the greatest, estimated at 200% increase over the 2013–50 time frame. The speed of ageing in both countries is higher than the average rate of increase in developed countries.^7,8

However, there are major differences in crude measures of ageing in these two countries, especially in life expectancy. Studying differences in indicators of old-age structure can inform policymakers on both sides of the Tasman Sea. The aim of this study was to calculate and compare five indices reflecting the differences in old-age structure between New Zealand and Australia, and to suggest plausible explanations for these differences.

Data from the World Bank, Australian Bureau of Statistics and New Zealand’s Ministry of Health, Ministry of Social Development and Statistics New Zealand were sourced for information about the disability, health and trends in older general and Indigenous populations.^9–13 The authors compiled results using the most recent data from each source.

Based on the publication by Deng et al.¹⁴ who analysed indicators that may best assess longevity, we calculated and compared five indicators of old-age structure in New Zealand and Australian older adults. The five longevity indicators are: centenarian ratio (number of centenarians per 100,000 people); LI; longevity level (proportion of the total population who are aged ≥80+/60+ years population); ageing tendency (proportion of the total population who are aged ≥60 years); and CeI. Table 1 shows all indices calculated.

Table 1. Indicators of old-age structure

Based on the World Bank dataset, the population, life expectancy and number of centenarians in Australia and New Zealand is 24.6 million population, 82.5 years life expectancy and 4279 centenarians in Australia and 4.8 million population, 81.6 years life expectancy and 450–500 centenarians in New Zealand.

All indices demonstrated an advantageous ageing structure in Australia compared with New Zealand. The disadvantages, that is, smaller indices, for older New Zealanders ranged from 5% to 14%, except for centenarians, who were even more disadvantaged. In addition, the Māori and Aboriginal ageing structure indices demonstrated a significant disadvantage to these ethnic groups compared with the ‘non-Māori’ and ‘non-Aboriginal’ data (Table 2). The disadvantages for ethnic minorities ranged from 10% to 60%.

Table 2. Indices of old-age population structure in New Zealand (NZ) and Australia

The degree of differences between countries was calculated by ratios of the proportions. New Zealand’s ratio of centenarians to the total population was 60.0% (95% confidence interval [CI]: 54.7–65.8%) of the same measure in Australia (P < 0.001), and the centenarians ratio as well as the CeI are the most disadvantaged indices compared with Australia.

This analysis of five indicators demonstrates a disadvantage of old-age structure in New Zealand. The demographic and health aspects of ageing populations in Australia and New Zealand were recently described by researchers who predicted that both countries will experience a >80% increase in the elderly population (people aged ≥60 years) by 2050.⁷ The average life expectancy at birth and age 60 years is higher in Australia than New Zealand, with the differences predicted to increase by 2050. However, these authors emphasised similarities rather than differences between the two countries – and these may inform strategies to change the trajectory of ageing in New Zealand. Internationally, data from Italy and Japan also report an old-age structure with higher longevity level and centenarians ratio or index. China reports a comparable level of longevity with New Zealand.^2,15

Several processes may account for the disadvantaged old-age structure in New Zealand. First, the health status of the Indigenous populations of both countries manifests as life expectancies substantially lower than the total populations. Premature adult mortality before the age of 60 years has improved for all populations, but mortality ratios show little change since 2000, with Aboriginal at nearly four-fold that of all Australians, and Māori nearly three-fold that of non-Māori.¹⁶ These findings may be attributed to nutritional inadequacy among older Māori¹⁷ and smoking rates that are more than twice the smoking rate in the general population;¹⁸ rates of dementia rapidly becoming a health priority in Australian Aboriginal communities;¹⁹ and high rates of diabetes and renal failure accounting for 28% of all Australian Aboriginal deaths.²⁰ Frailty is prevalent and emerging at a younger age than expected in remote-living Aboriginal Australians, with rates substantially higher than in other populations.²¹ Even after controlling for their differences in age structure, Māori and Pacific males had a higher relative risk of dying than European New Zealanders. Furthermore, ethnic mortality gradients persist into old age and the mortality level of most ethnic groups is influenced by erratic distribution of socioeconomic factors.²²

An additional difference between ageing in New Zealand and Australia is the significant differences in cardiovascular and cancer mortality. Cardiovascular disease rates are higher in New Zealand than Australia, and cardiovascular disease mortality is higher in New Zealand across all socioeconomic groups. In both countries, there were socioeconomic gradients in open-label usage of cholesterol-lowering medication, percutaneous coronary intervention and coronary artery bypass surgery. However, Australians in all socioeconomic groups were more likely than New Zealanders to receive these treatments.^23,24 From 2000 to 2007, there were each year 15% more deaths from cancer in New Zealand women than expected from Australian rates, and nearly 5% more deaths in men. Higher cancer death rates in New Zealand are not due simply to higher incidence.²⁵ The greater differences in deaths than in incidence suggest that patient survival is lower in New Zealand.²⁶ In addition, Australia showed significant improvements in overall 5-year cancer survival, whereas New Zealand had an only minimal increase in cancer survival.²⁷

We tentatively conclude that public health policy needs to target ageing in New Zealand as a major goal in advancing the ‘Ageing Well’ policy advocated by the government.²⁸ This will be in line with the Australian and New Zealand Society for Geriatric Medicine position statement including, among others, the call for ‘…further research into Māori Health’.²⁹

The authors declare no conflicts of interest.

This research did not receive any specific funding.