THE retirement of Pope Benedict XVI, the first such abdication in almost six centuries, may force ageing lions from politics, academia and business to confront that painful question. For the legions of leaders well into their 80s and beyond, the evidence isn't encouraging.
Pope Benedict's decision may prove emblematic of an older generation that is perhaps healthier than earlier generations, yet may harbour unrealistic expectations about what is physically possible as the body ages.
"The Pope is probably making a wise decision because his ability to totally manage the church is going to be more limited on average because he's 85," said Leo Cooney (69), chief of geriatrics at Yale School of Medicine in the US.
Many of the world's most influential business leaders are confronting the same decision as they continue to rule their empires in their 80s and even into their 90s.
Leaders in their 80s include Berkshire Hathaway's Warren Buffett (82); News Corp's Rupert Murdoch (81); BP Capital's T Boone Pickens J (84) and Tracinda's Kirk Kerkorian (95).
Viacom chairman Sumner Redstone and Berkshire Hathaway vice-chairman Charlie Munger are 89. Li Ka-Shing, chairman of Hutchison Whampoa, is 84.
The scientific community's consensus on these long-reigning tycoons is clear enough: all good things come to an end. Some of us get lucky and are vigorous into old age, but many others aren't.
"The prevalence of significant cognitive and functional problems does increase dramatically in the mid-80s," said Mr Cooney. "It's probably a good idea to reassess one's ability to be in a position of power at that time."
Life expectancy in the US is now 76 for men and 81 for women. It is about 78 in developed countries and 68 in developing regions, the United Nations said in a report last year.
By 2050, newborns can expect to live to 83 years in developed regions and 74 years in developing regions, it said. However, those fortunate to live beyond their ninth decade will face a myriad of difficult hurdles as they age.
By the time people reach their 80s, muscle mass decreases and so do strength and endurance.
Almost 70pc of Americans between the ages of 85 and 89 have a disability, defined as a substantial limitation in major life activity, according to a 2011 report on ageing by the US Census Bureau and Nat- ional Institute of Ageing.
"There's definitely not a magic number, but there are things that happen more as you age," said Audrey Chun (41), medical director of the Martha Stewart Centre for Living at Mount Sinai Medical Centre in New York. "As you start collecting these conditions where you have more than one chronic condition, those things can start taking a higher toll."
As conditions accumulate, so do the medications, which can lead to side-effects that can leave people somewhat impaired, Ms Chun said.
Dementia and cognitive decline also kick in. By 90, about half of people have some sort of cognitive problem, she said.
Difficulty in performing tasks alone and mobility-related activities, such as walking and climbing stairs, are the most common types of disability, affecting two-thirds of people aged 90 and older, according to the Census Bureau and National Institute of Ageing study. Hearing and vision loss affect 43pc and 26pc respectively.
But it's not all bad news, said Thomas Kirkwood, associate dean for ageing at Newcastle University in England, who has studied ageing for almost 40 years.
The university's Newcastle 85+ Study, which enrolled more than 1,000 85-year-olds from the Newcastle and North Tyneside areas, found that, on average, people had four or five age-related health conditions.
Still, about 80pc of them rated their health and quality of life as good or excellent, Mr Kirkwood said.
"I do feel some regret that the Pope's decision serves to confirm a negative stereotype we have in society about being old," he said in a telephone interview.
"The common idea that 85 is an age when someone is likely to be past it simply doesn't stand any more."
The findings suggest that women may face more hurdles then men in the mid and late- 80s. At 85, about 37pc of men reported no limitations to their daily living activities, such as cooking, bathing and managing personal finances. The percentage was about half that for women, Mr Kirkwood said.
"It's a very interesting paradox that although women live longer than men, women at the end of life experience more disability and ill-health than men," he said.
Some leadership roles have a tradition of abdication by a certain age. Queen Beatrix of the Netherlands, who is 75, will step down this year and be succeeded by her eldest son, Crown Prince Willem-Alexander, who will become the country's first king in more than a century. Beatrix succeeded her mother when she abdicated in 1980.
Most elderly powerful business people, though, have no intention of abdicating if they don't have to.
And that may be the smartest decision they ever make, according to many gerontologists. In fact, hanging on to a job full of responsibilities and challenges may be the best thing for an executive's health, said Barbara Messinger-Rapport, director of the Cleveland Clinic's Centre for Geriatric Medicine.
"In order to age successfully, you have to have cognitive challenges," she said. "You have to have social engagement, so you're with peers and have meaningful activities. You need physical challenges to maintain your ability."
Indeed, when it comes to ageing, no rules apply.
Michael DeBakey, the US cardiovascular surgeon who developed heart-bypass procedures that improved the lives of millions of patients and prolonged life for others, had a heart bypass at 97 and went back to work seven months later. He died, aged 99, in 2008.
Former South Carolina senator Strom Thurmond retired in 2003 and died later that year at 100, while his Democratic colleague, Representative John Dingell of Michigan, is the longest-serving member of the House at age 86.
"We need leadership in the whole business of getting older," said Mr Kirkwood.
He added: "Eighty-five these days is old, but it's not terribly old." (Bloomberg)