As much as I try to stay in the moment, I sometimes get obsessed with the future — as in, “How much time have I got left?” Not long ago, curious about this life-or-death question, I used the Social Security Administration’s life expectancy calculator to see how long I might live. Based on my age and gender, the calculator told me I’ve probably got another 22 years ahead of me, that is until I kick the bucket at 83. (Of course, an accident or a serious illness could ruin my calculation.)

Determining my life expectancy, it turns out, led to another conundrum that’s a frequent topic of conversation among my friends: Are we old? Typically, people decide who is “old” based on how many years someone has already lived, not how many more years they can expect to live, or even how physically or cognitively healthy they are. I will soon turn 62. What does that actually tell you? Not very much, which is why, like many of my sexagenarian friends, I’m apt to claim, “Yes, age is just a number.”

So what does “old” really mean these days?

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This isn’t an idle question — not only does the definition of “old” have an outsized impact on how we feel about ourselves (not to mention how others view us) it also matters to policymakers determining how to plan for aging populations.

The United Nations historically has defined older persons as people 60 years or over (sometimes 65). It didn’t matter whether you lived in the United States, China or Senegal, even though life expectancy is drastically different in each of those countries. Nor did it depend on an individual’s functional or cognitive abilities, which can also be widely divergent. Everyone became old at 60. It was as though you walked through a door at midnight on the last day of 59, emerging a completely different person the next morning: an old person.

Demographers Sergei Scherbov and Warren Sanderson at the International Institute for Applied Systems Analysis, who study aging, are evangelists about overturning the one-size-fits-all-across-the-globe definition of old. For nearly 15 years, they’ve been beating the drum that what they call “chronological age” (the number of years lived) is wrongheaded. In their forthcoming book, “Prospective Longevity: A New Vision of Population Aging,” they write that chronological age “tells us how long we’ve lived so far. In contrast, prospective age is concerned about the future. Everyone with the same prospective age has the same expected remaining years of life.”

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At a conference this past winter on population aging, I asked Scherbov the big questions: What makes someone old? It’s not when you turn 60 or 65, he replied, but when your specific life expectancy is 15 years or less. That, he says, is when most people will start to exhibit the signs of aging, which is to say when quality of life takes a turn for the worse.

My first reaction was to do a metaphoric triple axel and shout “hallelujah!” While it’s important to embrace every age — and not just youth — with a life expectancy of 83, or 22 years yet to live, I could honestly claim not to be old. That is, at least for another seven years.

Scherbov explained that young and old are relative notions, and their common reference point is life expectancy.

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“Some people may be old at 56, 60 or age 75,” he told me at the Bangkok conference.

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As an example, he asked me to imagine a 60-year-old woman in Japan, where life expectancy for women at 88 is the longest in the world; she shouldn’t be considered old until age 73. By contrast, a woman in Sierra Leone, the nation with the shortest life expectancy for women, at 72 years, is considered old at 57.

“These are very different people,” he says. “They have different life expectancies. . . . They have different cognitive abilities, different physical abilities.”

And how about folks in the United States, I asked? When are we considered old? For women, the old age threshold is about 73; for men, 70.

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Scherbov layers his concept of prospective age with another quality, which he calls “characteristic aging.”

“It depends upon the characteristics of people, in which sense they are old,” he says. “Are they cognitively old? Are they physically old? Are they old in terms of their disabilities? It depends.”

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With perfect hindsight, it makes sense that “old” would vary between nations, especially between more- and less-developed countries, with differences in education, mortality rates, access to health care and life expectancy.

But who is “old” also varies — widely — between individuals. Based on everything from their genes, diet and exercise habits, whether they’ve smoked and often their socioeconomic status. For kicks, I did an online search for “celebrities who are 70,” which included singers Ozzy Osbourne and Robert Plant. In photos, Osbourne looks much younger, which may be a result of better diet and genes, or more exercise and sleep.

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The point, says Scherbov, is that personal age is dependent on our “characteristics” — cognitive abilities, disability, health history and even education levels. Those with more education tend not to smoke, exercise more frequently, have better diets and have regular checkups — and, therefore, live longer, meaning their old-age threshold comes later, says Scherbov.

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Erwin Tan, a geriatrician who is director of thought leadership-health division at AARP, also wanted me to understand that “there are tremendous health disparities in the United States, which is why a Zip code is a very strong indicator of one’s life expectancy” and tends to reflect socioeconomic status. (The Robert Wood Johnson Foundation provides aZip code/life expectancy indicator here: www.bit.ly/life_­zipcode.)

I decided to try out Northwestern Mutual Life Insurance’s life span calculator, which digs deeper than the Social Security Administration’s version does. This tool asks 13 questions, including most of the characteristics Scherbov mentioned. It calculated that I’ll live until — drumroll, please — 93, which means I won’t cross the threshold to “old” until I’m 78. Playing with the variables the site provides, I could clearly see how one’s family history and personal lifestyle choices made a crucial difference to life expectancy. Eat your veggies! (plus-3 years); don’t exercise at all (minus-3 years); maintain a healthy blood pressure (plus-3); use drugs like cocaine or opioids (minus-8 years).

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Looking for one more sign of youth?

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Oddly enough, one of the best predictors of illness and even mortality is handgrip strength. Scherbov told me about a large British study that found, after accounting for age and a wide range of other factors, such as diet, sedentary time and socioeconomic status, that muscle weakness, defined by grip-strength, “was associated with higher overall risk of death and higher risk for specific illnesses.” Weaker grip strengths, the researchers say, were correlated with smoking, lower socioeconomic status, obesity, a diet low in fruits and vegetables, less exercise, and more TV viewing. Thus, longevity.

Now that I’m no longer officially “old,” I plan to hang on tight and live life to the fullest for as long as I can — so pass me those handgrips and stand back.

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