What’s your fitness age?

That’s a 21st-century question you can ask, thanks to the invention of the phrase fitness age. But what does this new term mean?

Here’s an answer provided by the lexicographer David Barnhart, editor of The Barnhart Dictionary Companion, a quarterly devoted to new words.

He defines fitness age as “a measure in years of age of a person’s physical condition and health relative to their chronological age, based on aerobic capacity and diet and stature.” He says that it’s frequently “used in technical contexts dealing especially with health.”

Like any careful lexicographer, Barnhart derives his definitions from the word as it is actually used — in this case, some half a dozen quotations. He first noticed it in an article by Gretchen Reynolds in The New York Times:

“I wrote last year about fitness age, a concept developed by researchers at the Norwegian University of Science and Technology, in Trondheim, who had taken note of epidemiological data showing that people with above-average cardiovascular fitness generally had longer life spans than people with lower aerobic fitness.”

That got him on the search for other instances. He likes Nexis, the venerable database of news publications. In Nexis he found an example going back as far as 1988, as well as this one from the Calgary Herald in 1996:

“He has jogged at least 43,470 kilometers in the past 20 years, and his 6-foot-1 frame is a wiry 170 pounds. He’s 65, but on his clinic’s scale he registers a ‘fitness age’ of 49.”

Barnhart found two other definitions for fitness age, one referring to cars and the other referring to ours as the fitness age, the latter with three quotations including this one from last year’s London Times:

“Maybe now, in the fitness age, somebody could say, ‘Wow, he was such a great long-distance walker.”

In past centuries, before the Internet, finding quotations was a challenge for a lexicographer. Nowadays there are plenty of examples instantly available through an Internet search. The challenge is finding quotations that give the most added information and the best examples of typical usage. Barnhart says he looks for quotations that have details like “location, date, celebrities, and components (in this case, exercise, cardiovascular function, and origin).”

That’s how a new locution displays its credentials in a dictionary.

And here’s how you can check your own fitness-age credentials. Barnhart refers us to a website, based on the research of the folks in Trondheim, that asks, How fit are you really?