The age-sex pyramid is a demographic tool which shows patterns of change in the age structure and sex ratio of an area over time. Its name comes from the traditional pyramid shape of an area which indicates a lot of children in large families, with a lower life expectancy and relatively few elderly. This shape is quite rare in developed nations like Australia. As nations and areas transition over time this shape turns into a column where there are less children a mix of ages including many elderly. Older areas with a lot of retirees or relatively few births may have a "reverse" pyramid formation where the largest proportion of the population is at the top of the chart. The sex ratio of males to females can also be seen â€“ young areas are likely to have more males, while elderly areas generally have more females due to longer life expectancy.