Sweden is the best place in the world to be old and Afghanistan the worst, according to a UN-backed global study.

The Global AgeWatch Index examined the quality of life of the elderly in 91 countries.

It warns that many countries do not have adequate support in place for their ageing populations.

By 2050, older people will outnumber children under 15 for the first time, with most of the elderly in developing countries, it said.

The Global AgeWatch Index was complied by the UN Population Fund and advocacy group HelpAge International, and released to mark the UN's Day of Older Persons.

Researchers used 13 different indicators - including income and employment, health provision, education, and environment - in what they said was the first study of kind to be conducted on a global scale.

The study's authors say countries across the world face an ongoing challenge from the rapidly ageing global population.

"The continual exclusion of ageing from national and global agendas is one of the biggest obstacles to meeting the needs of the world's ageing population," says Silvia Stefanoni, the interim chief executive of HelpAge International.

"By giving us a better understanding of the quality of life of women and men as they age, this new index can help us focus our attention on where things are going well and where we have to make improvements."

While Sweden came top, and Afghanistan was placed last, the top 20 was dominated by countries from Western Europe and North America, along with Japan, Australia and Chile.

Wealth was not the only factor considered, and countries such as Sri Lanka, Bolivia and Mauritius were ranked above several richer nations.

Some large fast-developing countries fared worse than others, with Russia (78), India (73) and Turkey (70) receiving a low rank, while Brazil (31) and China (35) were regarded more favourably by the researchers.