You may think you know how old you are, but your body doesn’t follow a calendar.

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That is, your chronological age, which measures how many years have passed since you were born, may not match your biological age, which reflects wear and tear on your body at a cellular level.

Based on factors including genetics, lifestyle habits and medical history, you may be biologically older or younger than your chronological age. That’s because these two measures don’t always progress at the same pace, according to Dr. Douglas Vaughan, director of the Potocsnak Longevity Institute at Northwestern University’s Feinberg School of Medicine.

“Every time you have a birthday, you add another year to your life,” Vaughan said. “We all experience that at the same rate, and it’s relentless, it’s unforgiving, it’s cosmically indifferent.”

On the other hand, biological age, also called epigenetic age, “reflects more of the changes inside you that occur over time,” he said.

Scientists have developed a number of ways to estimate biological age, including so-called epigenetic clocks that measure changes to DNA at the molecular level. These clocks, often created for clinical research purposes, are becoming mainstream in the form of direct-to-consumer test kits.

“We have lots of tools now that allow us to get some insight into a given individual’s biological age,” Vaughan said. “This whole field is exploding right now, as we learn that there are ways to measure biological age that we never even thought about before.”

As the longevity industry booms, so too does the biological age testing market, which reached $1.28 billion globally in 2024, according to market research and consulting firm Dataintelo. It’s projected to reach $3.09 billion by 2033.

Biological age test kits are widely available online, ranging from $299 saliva-based tests to $499 blood tests. But all tests aren’t created equally, and there are limitations to the health insights they provide.

“The measurement of epigenetic age is not prime-time yet for general consumers,” Vaughan said. “The data and the value of those kinds of tests is quite uncertain.”

What can epigenetic clocks tell us?

Epigenetic clocks are algorithms based on DNA methylation. This process, according to Daniel Belsky, an associate professor of epidemiology at the Robert N. Butler Columbia Aging Center at Columbia University, may be thought of as a dimmer switch that turns genes on and off.

Still, such clocks are largely engineered “without any reference to biology. It’s a purely statistical process,” Belsky said. “The labels for different epigenetic clocks are different, and they have very significant consequences for what that clock means.”

For example, a clock called PhenoAge estimates biological age. GrimAge — a clock named after the Grim Reaper — reflects the probability that a person will die within a year.

In 2022, Belsky was involved in developing a clock called DunedinPACE, which measures the rate of biological aging. (The clock is now exclusively licensed to TruDiagnostic, a company that sells biological age tests and for which Belsky is an occasional adviser.)

“We try to use the distinction of speedometer versus odometer,” Belsky said. “An odometer tells you how far you’ve traveled; that’s like your biological age. [DunedinPACE] is your speedometer, tells you how fast you’re going.”

A DunedinPACE value of 1 means a person is aging at the average rate, Belsky said. A score of 2 would indicate they’re aging twice as fast, and a score of 0 would mean they’re not aging at all. (In theory, a negative value would imply a person is getting younger.)

“Most people’s values fall between 0.5 and 1.5,” Belsky said. “Even a 10% difference in the face of aging is a big deal. If you only age 11 months for every 12 that you live, that’s not bad.”

Biological age is a snapshot

Steve Horvath, a professor of human genetics and biostatistics at UCLA’s David Geffen School of Medicine, helped develop the first epigenetic clock in 2011 using saliva.

Though it boasts a U.S. patent, “nobody uses this clock — nobody, including me,” Horvath said. “It just wasn’t good enough.”

However, his early efforts paved the way for epigenetic clocks in use today. In 2013, he went on to create the Horvath Clock, a timekeeper capable of analyzing various tissues and cell types, which is patented in Europe and China. The Regents of the University of California hold all patents associated with Horvath’s clocks.

Horvath also had a hand in inventing PhenoAge in 2018 and GrimAge in 2019. A number of over-the-counter tests use these markers to estimate biological age.

Horvath stressed that the primary use for these clocks remains in the laboratory. In the near future, though, he hopes testing may become a routine part of medical exams.

“The tests aren’t there yet because the clinical validation hasn’t caught up,” Horvath said. “However, very serious medical researchers are conducting tests to see whether there is a benefit.”

Considering how quickly longevity science is advancing, “it could be that I will change my mind in two years,” he added. “Who knows?”

Biological age is more than a number, according to Christopher Hine, the principal investigator of his own laboratory within Cleveland Clinic Research’s department of cardiovascular and metabolic sciences.

“It compares your current state of health — in terms of multiorgan systems, metabolism, weight, bone density — to your chronological aging,” Hine said. “It’d be the same thing thinking of the terms ‘lifespan’ and ‘health span.’ Lifespan is the number of years you live, but health span is the number of years you live without chronic diseases.”

No matter the type of biological age test, it represents a snapshot in time, Hine said. Something as simple as having a cold might influence your results.

“Only doing it once isn’t so telling, because you don’t know if you’re [on] an upward trajectory or downward trajectory in terms of the rate of your aging,” Hine said. “Having more snapshots of your epigenetic clocks is probably going to be better.”

Consider biological age comprehensively

While Hine anticipates biological age tests may play a larger role in medicine sooner rather than later, he cautioned that for the time being, they’re not intended to diagnose disease. Even taking a test on a whim isn’t risk-free, he said.

For example, a person who believes they live an active, healthy lifestyle might be shocked to learn their biological age is five years older than their chronological age and take drastic steps to try to improve it.

“There could be risk for individuals who become obsessed with bringing biological aging down,” Hine said. “You might want to go see a medical professional to follow up on it, as opposed to self-diagnosing or self-medicating, either through exercise or overdoing supplements.”

Chemical changes to DNA are malleable to a certain extent, said Vaughan, of Northwestern. For instance, research has shown that tobacco smoking is associated with faster biological aging, while taking daily multivitamins may slow biological aging in older adults.

Belsky also said consumers should interpret test results with caution, as it’s not uncommon for epigenetic clocks to disagree with one another.

“These are research tools that may provide some additional information to curious people,” Belsky said. “There are clocks that are better and worse at predicting future health outcomes.”

The most powerful predictor of any disease is chronological age, according to Vaughan, a cardiologist by training. Now, biological age may be an even better one, he said.

“It’s really refined and improved our ability to think about what puts people at risk for a variety of different diseases as we age,” Vaughan said. However, considering biological age alone “can be misleading and not provide real guidance to someone about their health and their future well-being.”

Resist jumping to conclusions about biological age test results, Vaughan said — especially if they’re positive.

“If you do a test and you find out that your biological age is 10 years younger than your chronological age, well, that makes for great cocktail party conversation,” Vaughan said. “But what’s your cholesterol? What’s your blood pressure? If those kinds of things aren’t addressed, you’re going to have a problem.”