Decomposition: population ageing at a sub-national level using Tasmania, Australia as a case study

While population ageing is considered a ‘major concern’ by over half the governments worldwide according to the UN and ‘the most important population issue for the ensuing two decades’ by a 2012 survey of leading demographers, research into population ageing and its causes at a sub-national level has only emerged more recently. This paper introduces a model for identifying the factors influencing the change in the mean age of a population. Through decomposition the model identifies how, and to what extent, each component of demographic change (fertility, mortality, overseas migration and interstate migration) contributes to the overall change in the mean age of the population over a given period. The paper uses the State of Tasmania, Australia to quantify, for the first time, the difference between national and sub-national population ageing. It finds that from 2005 to 2015, Tasmania’s predicted mean age increased by an average of 0.222 years per annum compared with 0.108 years per annum for Australia. This differential of 0.114 years is largely explained by the ageing effect of interstate migration, averaging 0.073 years per annum, which is not relevant at a national level. The paper confirms that as regional differences between fertility and mortality levels within a nation are usually small, it is migration which is predominantly responsible for different changes in the mean age of the population between national and subnational levels.