“No age jokes tonight, all right?” quipped Sir Mick Jagger, the 73-year-old front man of the Rolling Stones, as he welcomed the crowds to Desert Trip Music Festival in California last October. The performers’ average age was just one year below Sir Mick’s, justifying his description of the event as “the Palm Springs Retirement Home for British Musicians”.

Globally, a combination of falling birth rates and increasing lifespans will increase the “old-age dependency ratio” from 13% in 2015 to 38% by the end of the century. To listen to the doomsayers, this could lead not just to labour shortages but to economic stagnation, asset-market meltdowns, huge fiscal strains and a dearth of innovation.

From the start of the 20th century survival rates in old age started to improve markedly, a trend that continues today. The UN estimates that between 2010 and 2050 the number of over-85s globally will grow twice as much as that of the over-65s, and 16 times as much as that of everyone else.

Warnings about a “silver time bomb” or “grey tsunami” have been sounding for the past couple of decades, and have often been couched in terms of impending financial disaster and intergenerational warfare. Barring a rise in productivity on a wholly unlikely scale, it is economically unsustainable to pay out generous pensions for 30 years or more to people who may have been contributing to such schemes only for a similar amount of time. But this special report will argue that the longer, healthier lives that people in the rich world now enjoy can be a boon, not just for the individuals concerned but for the economies and societies they are part of. The key to unlocking this longevity dividend is to turn the over-65s into more active economic participants.

This starts with acknowledging that many of those older people today are not in fact “old” in the sense of being worn out, sick and inactive. Yet in most countries the age at which people retire has barely shifted over the past century. When Bismarck brought in the first formal pensions in the 1880s, payable from age 65, life expectancy in Prussia was 45. Today in the rich world 90% of the population live to celebrate their 65th birthday, mostly in good health, yet that date is still seen as the starting point of old age.

This year the peak cohort of American baby-boomers turns 60. As they approach retirement in unprecedented numbers, small tweaks to retirement ages and pensions will no longer be enough. This special report will argue that a radically different approach to ageing and life after 65 is needed.

The closest meaning with the word “boon” in the fourth paragraph is a (n)________.