GENEVA, Switzerland - The explosion in the world's ageing population presents herculean challenges for health systems, especially in poorer countries, the UN's World Health Organisation (WHO) warned on Thursday.
As people live longer, the need to care for long-term, age-related health problems will grow, it said.
"Worldwide, life expectancy of older people continues to rise," the WHO said in a press release.
"By 2020, for the first time in history, the number of people aged 60 years and older will outnumber children younger than five years.
"By 2050, the world's population aged 60 years and older is expected to total two billion, up from 841 million today."
The agency added: "Eighty per cent of these older people will be living in low- and middle-income countries."
The WHO, in an overview published in the British health journal The Lancet, said a longer lifespan was not necessarily associated with good health.
A large percentage of the global burden of illness is concentrated in people aged over 60.
Much of this takes the form of long-term illnesses such as cancer, respiratory disease, arthritis and mental and neurological disorders.
The number of people with dementia, for instance, is expected to rise from 44 million now to 135 million by 2050, the WHO said.
The ageing phenomenon required "deep and fundamental reforms" of health and social care systems, it added.
The issue "is the next top of the agenda," said Sommath Chatterji of the WHO's Department of Health Statistics, arguing that measures did not have to be costly to be effective.
Affordable options included mobile clinics for rural populations, improving vaccine coverage and advice for healthy living, reducing salt intake and tobacco use, and allowing people who are fit to remain in the workforce for longer.