The shape of an age-sex pyramid illustrates a nation's birth rates, death rates, and demographic composition. A triangular age-sex pyramid signifies high birth and mortality rates, common in developing countries with a predominantly young population, resulting in a broad base that narrows rapidly.
Evaluating the options:
- Australia: Typically exhibits a more uniform age-sex distribution, not a triangular shape. Developed countries such as Australia tend to have a column-like or slightly expanding pyramid, reflecting lower birth rates and increased life expectancy, leading to more balanced age groups compared to the sharp taper of triangular pyramids.
- Bangladesh: Frequently presents a triangular pyramid due to elevated birth rates and a youthful demographic.
- Nigeria: Consistently shows a triangular shape, attributed to a high percentage of young individuals, indicative of high birth rates.
- Mexico: Historically, Mexico has displayed a triangular age-sex pyramid, though it is progressively shifting towards a bell-shaped or barrel-shaped form as birth rates decline and life expectancy rises.
Therefore, Australia is the country that does not possess a triangular shaped age-sex pyramid.