Two demographers argue in an upcoming book that conventional definitions of "old age" are too inflexible and fail to recognize the big differences in how individual people age, Steven Petrow reports for the Washington Post.

Why 'chronological age' may misrepresent your age

Historically, the United Nations has defined an "older" person as anyone 60 years or older, regardless of that person's individual history or where in the world they live. "Everyone became old at 60," Petrow writes. "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."

However, Sergei Scherbov and Warren Sanderson, demographers at the International Institute for Applied Systems Analysis, say that one-size-fits-all approach is misguided, and that any meaningful definition of old age must take into account each person's location and lifestyle.

In their upcoming book, "Prospective Longevity: A New Vision of Population Aging," Scherbov and Sanderson argue that "chronological age," or the number of years a person has lived, "tells us [only] how long we've lived so far."

In order to determine whether that person qualifies as "old," Scherbov claims, it's better to look to their "prospective age"—or how many years of life expectancy they have remaining.

What does 'prospective age' reveal about old age?

When it comes to prospective age, a person should be considered "old" when their life expectancy is 15 years or less, Scherbov argues.

That "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," Petrow writes.

"Some people may be old at 56, 60, or age 75," Scherbov said—with the exact pivot point into old age depending on their personal life expectancy, which varies between individuals and across countries.

As an example, Scherbov said, imagine a typical 60-year-old woman living in Japan, where women have the highest life expectancy in the world, at age 88. That woman shouldn't be considered old until she's 73, Scherbov said.

In contrast, a typical woman living in Sierra Leone, where the life expectancy for women is the shortest in the world at 72, would be considered old at age 57.

"These are very different people," Scherbov said. "They have different life expectancies. … They have different cognitive abilities, different physical abilities."

By that definition, a typical woman in the United States is old at age 73, and a typical man at age 70, Petrow writes.

A third type of old age: 'Characteristic aging'

The term "prospective aging" works in conjunction with the concept of "characteristic aging," according to Petrow—that is, whether a person displays characteristics conventionally associated with old age, such as a decline in cognitive and physical function.

The onset of these characteristics can happen sooner or later depending on a person's genes, diet and exercise habits, socioeconomic status, and whether they've smoked, Petrow writes.

What ZIP code you live in in the United States also influences your life expectancy, according to Erwin Tan, a geriatrician and director of thought leadership-health division at AARP. Tan said 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" (Petrow, Washington Post, 6/17).

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