We’ve heard the old adage that you’re only as old as you feel. Now researchers are saying there might be some truth to the popular saying after all.

A study published today in “Population and Development Review” suggests age really isn’t just about the number of years you’ve been around. “Your true age is not just the number of years you have lived. It also includes characteristics such as health, cognitive function, and disability rates,” researcher Sergei Scherbov said in a release.

Demographers have traditionally relied on chronological age in studies of aging, but Scherbov and co-author Warren Sanderson say using other biological characteristics to define age can help provide a better understanding of the aging process.

A 2009 study from Denmark found that younger-looking twins tend to outlive their older-looking counterparts, showing that perceived age is more important than your date of birth. And with a rapidly growing older population, understanding aging is important now more than ever.

While the current average U.S. life expectancy is 78.7 years, according to the CDC, while just over 50 years ago it was around 60 for men and 70 for women. “We used to consider people old at age 65. Today, someone who is 65 may be more like someone who was 55, 45 years ago in terms of many important aspects of their lives,” Scherbov said. “Aging is multidimensional.”