It has made "active ageing" the focus of this year's World Health Day which is celebrated on Wednesday.

The organisation is seeking to dispel a number of myths about old age, including the idea that most elderly people live in the industrialised countries of the north.

Dr Gro Harlem Brundtland, director general of the WHO, said: "The ageing of the global population is one of the biggest challenges facing the world in the next century.

"It is also potentially a great opportunity. Older people have a lot to contribute.

"Older people are often viewed as a homogeneous group from mainly industrialised countries, who no longer contribute to their families and societies, and may even be a burden. The truth could not be more different."

Fast growth rate

The WHO says more than 60% of elderly people live in developing countries.

There are currently about 580 million older people in the world, with 355 million in developing countries.

By 2020, it predicts there will be 1,000 million, with over 700 million in the developing world.

The rate of growth is extremely fast in some countries.

In China, for example, the proportion of elderly people is expected to double between the year 2000 and 2027 - from 7% to 14%.

In France, a similar increase took 115 years.

The rising numbers of elderly are a result of medical and social advances which have seen a fall in death rates from infectious diseases and improvements in sanitation, housing and nutrition.

The changes are predicted to cause a sharp increase in the numbers of chronic diseases in many countries.

The WHO says chronic diseases including cardiovascular diseases, diabetes and cancer are predicted to be "the main contributors to the burden of disease" in developing countries by 2020.

In many developing countries fertility rates are also falling, meaning that the proportion of older people is likely to exceed that of children and teenagers by 2050.

'Active ageing'

However, the WHO says better healthcare means a large number of these elderly people may enjoy many years of healthy active life.

It is encouraging people to change their attitudes to old age and says improving health is key to "active ageing".

China's elderly population is expected to boom in the next century

Ms Harlem Brundtland said: "There is much the individual can do to remain active and healthy in later life.

"The right lifestyle, involvement in family and society and a supportive environment for older age all preserve well-being."

The WHO says elderly people have a lot to contribute to society.

In the US, for instance, over three million people aged over 65 take part in voluntary activities in schools and religious, health and political organisations.

It is estimated that more than two million American children are cared for by their grandparents.

The WHO says many countries in Asia and Africa perceive the elderly in a positive way as "people with knowledge".

But it warns that in some cultures these traditional values are in danger of being eroded.

Later this year - the International Year of Older Persons - the WHO is launching a Global Movement for Active Ageing, a network for all those interested in promoting active ageing.