The world is about to cross a demographic landmark of huge social and economic importance, with the proportion of the global population 65 and over set to outnumber children under five for the first time.

A new report by the US census bureau highlights a huge shift towards not just an ageing but an old population, with formidable consequences for rich and poor nations alike. The transformation carries with it challenges for families and policymakers, ranging from how to care for older people living alone to how to pay for unprecedented numbers of pensioners – more than 1 billion of them by 2040.

The report, An Ageing World: 2008, shows that within 10 years older people will outnumber children for the first time. It forecasts that over the next 30 years the number of over-65s is expected to almost double, from 506 million in 2008 to 1.3 billion – a leap from 7% of the world's population to 14%. Already, the number of people in the world 65 and over is increasing at an average of 870,000 each month.

The rate of growth will shoot up in the next couple of years, with both overall numbers and proportions of older people rising rapidly.

The shift is due to a combination of the time-delayed impact of high fertility levels after the second world war and more recent improvements in health that are bringing down death rates at older ages. Separate UN forecasts predict that the global population will top 9 billion by 2050.

The US census bureau has led the way in sounding the alarm over the changes. This is its ninth report drawing together data from around the globe since it first focused on the trend in 1987.

Its latest projections warn governments and international bodies the tipping point will present widespread challenges at every level of human organisation, starting with the structure of the family, which will be transformed as people live longer.

That will in turn bring new burdens on carers and social services providers, while patterns of work and retirement will similarly have huge implications for health services and pensions systems.

"People are living longer, and in some parts of the world, healthier lives," the authors conclude. "This represents one of the crowning achievements of the last century but also a significant challenge as proportions of older people increase in most countries."

Europe is the greyest continent, with 23 of the world's 25 oldest countries. Such dominance of the regional league table will continue. By 2040, more than one in four Europeans are expected to be at least 65, and one in seven at least 75.

The UK comes in at number 19 in the list of the world's oldest countries. Top of the pile is Japan, which recently supplanted Italy as the world's oldest big country. Its life expectancy at birth – 82 years – is matched only by Singapore, though in western Europe, France, Sweden and Italy all have life expectancies of more than 80 years (in the UK it is 78.8).

The contrast in life expectancy between rich and poor nations remains glaring. The report shows that a person born in a developed country can expect to outlive his or her counterpart in the developing world by 14 years. Zimbabwe holds the unfortunate record for the lowest life expectancy, which has been cut to 40 through a combination of Aids, famine and dictatorship.

But an important finding of the report is that the wave of ageing that has until recently been considered a phenomenon of the developed world is fast encroaching on poorer countries too. More than 80% of the increase in older people in the year up to July 2008 was seen in developing countries.

By 2040, the poor world is projected to be home to more than 1 billion people aged 65 and over – fully 76% of the world total.

Ageing will put pressure on societies at all levels. One way of measuring that is to look at the older dependency ratio, or ODR, which acts as an indicator of the balance between working-age people and the older population that must be supported by them.

The ODR is the number of people aged 65 and over for every 100 people aged 20 to 64. It varies widely, from just six in Kenya and seven in Bangladesh, to 33 in Italy and also Japan. The UK has an ODR of 26, and the US has 21.

From that ratio, a number of profound challenges flow. Countries with a high ODR are already creaking under the burden of funding prolonged retirement for their older population. Life expectancy after retirement has already reached 21 years for French men and 26 years for French women.

Though retirement ages have begun to rise in developed countries, partly through inducements from governments to continue working, this still puts an extreme burden on public pensions funds.

Socially, too, there are intense pressures on individuals and families.

With women living on average seven years longer than men, more older women are living alone. Around half of all women 65 and over in Germany, Denmark and Slovakia are on their own, with all the consequent issues of loneliness and access to care that ensue.