How old is the person trying to open this jar? (Fred Barbash/The Washington Post) How old is the person trying to open this jar? (Fred Barbash/The Washington Post)

Age was always an artificial construct.

What difference, really, does it make how many times the Earth has revolved around the sun during your lifetime? A 60-year-old in a poor village in India is likely to be a lot “older” than someone the same age in an affluent community in the United States when it comes to things that matter, like how much longer and more comfortably he or she might live.

“You’re as young as you feel,” while uplifting, isn’t very useful either because it’s kind of hard to measure.

Try instead: “You’re as young as your grip” — as in hand-grip.

“The strength of your grasp,” say two researchers in the latest study on the subject, “may also be one of the most useful ways to measure your true age.”

It is, they write, “consistently a good indicator of future mortality and susceptibility to disease.”

Warren C. Sanderson of Stony Brook University and Sergei Scherbov of the International Institute for Applied Systems Analysis in Austria report their findings in an article called “Measuring the Speed of Aging across Population Subgroups,” published in the journal PLOS One, on which they elaborate in the Conversation under the more catchy title, “Really, how old are you? The hands never lie.”

They are not the first to reach this conclusion — just the latest. They’ve accumulated plenty of evidence from their own studies and those of others to back it up up.

In one study they cite, “555 people in Leiden, Netherlands who were 85 years old when enrolled between September, 1997 and September, 1999 were followed, as they survived, through February, 2008 … The main result was that study participations with relatively low hand-grip strength at age 85 had a statistically significantly higher mortality rate from all causes.”

In another study, 558 men older than 75 in a nursing home in Taiwan were followed for 3 years. “Low hand-grip strength was statistically significantly associated with the risk of infection-related death,” the researchers reported.

In the largest study, more than a million Swedish adolescent males born between 1951 and 1976 were followed. The conclusion: “Lower hand-grip strength, particularly for those with hand-grip strengths below the median, were significantly associated with higher all-cause mortality, higher mortality from cardiovascular disease, and a higher risk of suicide.”

Their own study investigated the relationship between hand grip and education. They found that more educated white men and women older than 69 had the same average hand-grip strength as 65-year-olds with less education, indicating that the less-educated group had aged faster.

“Hand-grip strength,” they write in the journal, “has been shown definitively to predict poor outcomes in a wide variety of mortality, morbidity, and other health outcomes such as lengths of stay in hospitals or rehabilitation centers. The associations have been demonstrated for both younger and older people, for community-dwelling populations and those in institutions, and for people in many different countries. Because of these associations with a wide variety of characteristics that are associated with aging, hand-grip strength is a useful metric for assessing how fast sub-groups of a population have aged.”

They note that hand-grip strength is one of several standard measures of health used by the National Institute of Aging’s “Health and Retirement” survey.  The measurement, in pounds or kilograms, is made using something called a spring-type hand dynamometer.