(iStock)

“You look young for your age.” Aside from being flattering, the sentence also highlights the fact that we can inhabit two ages at once: chronological age and biological age. Chronological age dictates the number of birthday candles we blow out every year, while biological age is a measure of our physiological state compared to other people with the same number of annual growth rings.

“It’s not all that helpful to talk about chronological age,” says Laurie Archbald-Pannone, a physician who specializes in geriatrics at the University of Virginia Health System. “It doesn’t tell us how resilient the body is.” To put it another way: Chronological age has very little to do with our actual physical well-being.

For example, a 50-year-old smoker can have the lung capacity of an 80-year-old, says Todd Miller, associate professor in exercise and nutrition sciences at George Washington University. “In other words, the 50-year-old smoker has the ‘lung age’ of an 80-year-old,” Miller says.

But it’s not just active misuse of the body — such as smoking — that ages us beyond our chronological age. It’s also ­— perhaps even more commonly — inactivity, says Miller, whose lab measures health and performance-related markers such as bone density, body composition and max oxygen uptake. “Many of the things that we consider ­age-related changes are more related to activity or inactivity as opposed to age,” Miller says. For example, “weight gain is not obligatory with age.”

If age holds only so much of our fate in its grip, that gives us more reason to incorporate healthy habits into our routines. Although age-related changes are inevitable, they “can be mitigated by sleep, exercise, nutrition and stress reduction,” Miller says. Exactly how much can be mitigated bodywide, he says, is unclear. But in terms of specific markers — such as lung capacity — our habits (healthy or not) can change our biological age by several decades.

And just to be clear, biological age is as much about feel and function as it is about looks. “How we function on a daily basis is what matters the most,” says Archbald-Pannone, referring to her (chronologically) aging patient population.

Elizabeth Brooks, a D.C.-based personal trainer, says her wellness regime (good rest, nutrition, community, fitness) makes her feel much younger than her 51 years. Many people assume it’s in her genes to look young and be in shape. Brooks was once overweight, something that runs in her family. “It’s that concept of nature and nurture,” she says. “The majority of who we are is nurture.”

Eyleen O’Rourke, a professor of biology and cell biology at the University of Virginia, agrees. “Lifestyle matters a lot for aging,” O’Rourke says. “DNA damage caused by things like smoking and metabolic syndrome accelerate what we think of as age-related illnesses,” she says. Metabolic syndrome is a cluster of conditions such as high blood sugar, high blood pressure and extra body fat around the stomach. It is known to increase risk for conditions such as stroke, diabetes, heart disease and cancer — though the mechanism is not yet understood. Although we might not know why metabolic syndrome is associated with higher rates of cancer, “we do know that excess fat reduces the natural rejuvenation of cells,” O’Rourke says.

And there is the crux. The ability of cells to rejuvenate, to respond to damage, is what keeps us “young.” Good genes help, but good habits help more. “Your genome isn’t determinative. It doesn’t have strong predictive power by itself,” O’Rourke says. “Everything is context. And there is hope for everyone.”

The earlier we start, the better. Bone growth is at its prime in childhood and adolescence; and people who train and eat healthily throughout their lives will be stronger at age 70 or 80 than those who take up the healthy habits later. That said, it’s never too late to start, Miller says. “Muscle responds at any age. You can get positive results even if you start lifting weights at age 80.”

Boston is a fitness trainer and freelance writer. She can be found at gabriellaboston.com.