A "senior citizen" is generally thought to be 65 or older, but a new study contends that it might be just the start of middle age.

Traditional studies that categorize people according to age tend to use only one measurement, their chronological age. But this is misleading, say researchers. Other factors should be taken into consideration, especially the number of years they are expected to live.

In the new study, researchers compared the proportion of the population that was categorized as "old" using the conventional measurement of age 65 to those considered "old" based on their new measure of age, which incorporates changes in life expectancy.

“Two hundred years ago, a 60-year-old would be a very old person, but someone who is 60 years old today I would argue is middle-aged, said Sergi Scherbov, population researcher and co-author of the new study, which was published in PLOS One.

In their previous work, the study’s authors have contended that not only chronological age, but also health, cognitive function, and other measures should be used by demographers to better understand aging.