THE world is on the brink of a demographic milestone, according to the World Health o rganization (WHO).

Since the beginning of recorded history, people aged 65 or older will outnumber children under age five. Driven by falling fertility rates and remarkable increases in life expectancy, population ageing will continue, even accelerate. The number of people aged 65 or older is projected to grow from an estimated 524 million in 2010 to nearly 1.5 billion in 2050, with most of the increase in developing countries.

The remarkable improvements in life expectancy over the past century were part of a shift in the leading causes of disease and death. At the dawn of the 20th century, the major health threats were infectious and parasitic diseases that most often claimed the lives of infants and children. Currently, non-communicable diseases (NCDs) that more commonly affect adults and older people impose the greatest burden on global health.

The potential economic and societal costs of NCDs of this type rise sharply with age and have the ability to affect economic growth. A WHO analysis in 23 low-and middle-income countries estimated the economic losses from three NCDs (heart disease, three stroke, and diabetes) in these countries would total US$83 billion between 2006 and 2015.

Reducing severe disability from disease and health conditions is one key to holding down health and social costs. The health and economic burden of disability also can be reinforced or alleviated by environmental characteristics that can determine whether an older person can remain independent despite physical limitations. The longer people can remain mobile and care for themselves, the lower are the costs for long-term care to families and society.

In the meantime, generations of children and young adults who grew up in poverty and ill-health in developing countries will be entering old age in coming decades, potentially increasing the health burden of older populations in those countries.

With continuing declines in death rates among older people, the proportion aged 80 or older is rising quickly, and more people are living past 100. The limits to life expectancy and lifespan are not as obvious as once thought. And there is mounting evidence from cross-national data that — with appropriate policies and programmes — people can remain healthy and independent well into old age and can continue to contribute to their communities and families.