The world is ageing so fast that most countries are not prepared to support their swelling numbers of elderly people, according to a global study going out Tuesday by the United Nations and an elder rights group.

The report ranks the social and economic well-being of elders in 91 countries, with Sweden coming out on top and Afghanistan at the bottom. It reflects what advocates for the old have been warning, with increasing urgency, for years: Nations are simply not working quickly enough to cope with a population graying faster than ever before. By the year 2050, for the first time in history, seniors over the age of 60 will outnumber children under the age of 15.

Aging is an issue across the world. Perhaps surprisingly, the report shows that the fastest aging countries are developing ones, such as Jordan, Laos, Mongolia, Nicaragua and Vietnam, where the number of older people will more than triple by 2050. All ranked in the bottom half of the index.

The Global AgeWatch Index (www.globalagewatch.org) was created by elder advocacy group HelpAge International and the UN Population Fund in part to address a lack of international data on the extent and impact of global aging. The index, released on the UN’s International Day of Older Persons, compiles data from the UN, World Health Organization, World Bank and other global agencies, and analyzes income, health, education, employment and age-friendly environment in each country.

The report fits into an increasingly complex picture of aging and what it means to the world. On the one hand, the fact that people are living longer is a testament to advances in health care and nutrition, and advocates emphasize that the elderly should be seen not as a burden but as a resource. On the other, many countries still lack a basic social protection floor that provides income, health care and housing for their senior citizens.

Many governments have resisted tackling the issue partly because it is viewed as hugely complicated, negative and costly — which is not necessarily true, says Silvia Stefanoni, chief executive of HelpAge International. Japan and Germany, she says, have among the highest proportions of elders in the world, but also boast steady economies.

“There’s no evidence that an aging population is a population that is economically damaged,” she says.

Prosperity in itself does not guarantee protection for the old. The world’s rising economic powers — the so-called BRICS nations of Brazil, Russia, India, China and South Africa — rank lower in the index than some poorer countries such as Uruguay and Panama.