Australia has passed through the demographic transition. This means that an older population is inevitable. But we should distinguish between the normal demographic ageing that is a consequence of this transition and the hyper demographic ageing which will result from very low fertility. If the total fertility rate can be raised to between 1.85 and 2.1 the age structure that will result by 2051 will be manageable, even beneficial. If the total fertility rate should fall to 1.6 (or below) the proportion aged 65 plus will be relatively large and the population as a whole will risk exponential decline. It is fertility that shapes these different scenarios: immigration has a considerable effect on the overall size of the population but a minimal effect on its age composition. Contemporary data also suggest that, after early childhood, biological age is only loosely associated with physical dependence. Social dependency, in contrast, is high at all age levels. However, as this is created by social institutions it is within our power to modify it if we wish. We cannot avoid the ageing of the population. But we should try to ensure that we reach a state of normal ageing rather than hyper ageing, and that we provide opportunities for adults of all ages to contribute to society.