Box 6: Tasmania's age-sex structure
A population's age-sex structure is the number and/or proportion of the population to be found in each age-sex group. If each population could be got together for a day and lined up in their age groups - females at one end, males at the other, a plane flying overhead would look down on a certain shape.
As recently as the 1970s, Tasmania's age structure was triangular in shape, with most of the population in the younger age groups. Over the past two decades the shape has become something like an apple core (or hourglass), with a bite out of each side around the 18-38 year age groups. The deep bite over the 18-38 year age groups and the resulting rapid shift from a younger to an older age structure since just 1991 can be seen in the accompanying graph.
Of the two factors that typically have most influence on age structures (changes in births and life expectancy) the main cause of the shift from a young to an old age structure is normally a decline in the birth rate. Low birth rates result in a decline in the proportion of the population to be found at younger ages, and an increase in the proportion at older ages. The phenomenon is known as structural ageing. Increasing life expectancy at older ages is also involved, in that it adds to the numbers and proportions of elderly, but it is not the primary cause of structural ageing - a population can experience an increase in the number of elderly but not necessarily an increase in proportion, if fertility is high (this occurred during the baby boom). However, soon (from around 2010), the baby boomers born in the high fertility years 1945-1961 will move into the older age groups, and will then greatly increase both the numbers and proportions at older ages.
Tasmania has recently become Australia's oldest state, with its median age passing that of South Australia. However, this is not due only to low fertility (Tasmania's birth rate has seldom been below 2.1), but rather is due to the bite in the key reproductive age groups. Even if Tasmania's birth rate (per woman) were to increase further (and it should be noted that globally Tasmania's birth rate is quite high, and in Australia is second only to that of the Northern Territory), the resulting number of births would have little impact on the age structure in the short to medium term. It goes without saying that even a high birth rate will do little for a population's age structure if there are only a small proportion of men and women at reproductive age, as the accompanying graph illustrates is the case.