Young at heart: Kobie Keenan, right, with daughters Maya and Sienna, and husband Justin. "I don't see myself as being 37, though. I still think of myself as wanting to be a little bit irresponsible," she said, laughing. They say age is just a number but median age tells demographers a lot about a country. Australia stands among the world's oldest countries by this measure, with a higher median age than three-quarters of countries. The world's median age is 30. In Niger, the world's youngest country, you become part of the country's older half of the population when you turn 16.

If that makes you feel old, head to Monaco, the world's oldest country, where you get to be in the population's younger half for an impressive 52 years. Increasing life expectancy and declining fertility rates and infant mortality mean median ages across the globe are rising, according to Australian Bureau of Statistics demographer Andrew Howe. "Look at places like Bangladesh, Pakistan, some of the subcontinental countries. In the 1970s, seven or eight children was the average; now you're talking three of four," he said. In Australia, the median age is rising, but at a slower rate than in the past. It has increased by three years over the past 20, compared with a five-year increase over the 20 years before that, Mr Howe said. And in 40 years' time, Australia's median age is projected to be about 41. Population age structure varies enormously around the world, even between countries with similar median ages.

Each distinct shape, as the charts below show, is like "a little lesson in history", Mr Howe said. Take Russia, for example. The trough among 70- to 74-year-olds shows the effect of the low fertility caused by the chaos World War II. The shallower trough among 45- to 49-year-olds is an 'echo' of the first, representing the second generation of children never born. The post-war baby boom, which began in the late 1940s, can be seen in the charts for Australia, the US, UK, Russia and Germany, in the bulge in population aged 45-69. The twin peaks in the Japanese chart reflect the country's two short post-war booms in the late 1940s and early 1970s. The overall shape of country's age structure is driven by fertility (which mainly influences the start of the graph), mortality (which shapes its tail) and, in some countries, immigration (which affects the middle, since most people immigrate as young adults), said James Raymer, head of the Australian National University's School of Demography.

Sharp bumps in a country's curve usually reflect policy changes that affect the fertility rate, he says. In China's chart, for example, the one-child policy, initiated in 1979, explains the dip in population aged 30-39. The deep trough at the start of the graph, showing the low number of Chinese children, reflects the policy's effect over two generations. "When we look at the graph for Niger, we see that a huge proportion of its population are children. That's saying it's a high fertility country … experiencing exponential population growth," he said. Developing countries tend to have far younger populations than developed countries, a pattern related to the shift from traditional, agriculture-based societies to modern, urban and more educated societies, Professor Raymer said. This "wealth flows" theory was developed by internationally-renowned ANU demographer Jack Caldwell, who died in March this year. "In traditional societies, stereotyped by sub Saharan society today, children are a resource. The more children you have, the better off you are because they can work on farms, gather food and help out around the place," he said.