Your driver’s license states the hard truth of chronological age: Even if you still feel like you did in your 40s, you were born 52 years ago. Then again, your wearable fitness tracker says your biological age is 45, but a blood test you paid for a few months ago puts that number at 54.

In the modern world of health measurement, the age-old measuring data points have become squishier. But what exactly is biological age? Which numbers matter, and what do they mean? Most importantly for those studying these data points: Are they helpful tools for monitoring health and staying more youthful?

Those questions are central to a fast-growing industry built around “biological age” – the idea that you have an internal, measurable age different from the number of years you’ve been alive.

Recently, commercial direct-to-consumer tests have hit the market promising to tell you this number, often coupled with advice on how to lower your biological age or prevent it from creeping up. The tests take a variety of approaches: Some measure metabolic markers; others look directly at DNA or analyze proteins in your blood.

But even researchers who design new ways to detect biological aging question the value of these tests.

“Right now, it’s a bit like the Wild West,” said Tony Wyss-Coray, PhD, a professor of neurology and neurological sciences and the D. H. Chen Professor II, who has spent his career studying brain aging. “There are all these different companies advertising tests, often with some kind of powder they want to sell you that’s never been tested in humans in a clinical trial.”

There are no direct-to-consumer biological age clocks that are approved by the U.S. Food and Drug Administration as medical diagnostic tools. Instead, they are sold as wellness or informational products. But the concept of a biological clock is, indeed, based on decades of research – and it is being used actively by scientists to study aging.

What is biological age, and is there anything a consumer test can tell you about it? We asked Wyss-Coray and other Stanford Medicine researchers about what the science does and doesn’t support.

What is biological age?

Biological age is a measure of how a person’s body – made up of their internal organs and cells – is changing with time.

“If you compare several 60-year-olds, some of them might be destined to live to 100 years old, and they might have a younger biological age than average,” explained Anne Brunet, PhD, professor of genetics and co-director of the Paul F. Glenn Center for the Biology of Aging Research. “Others are destined, unfortunately, to maybe die in a couple of years and might have a much older biological age.”

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At its root, biological age is based on the idea that all the cells in our bodies change in predictable ways. Scientists hypothesize that the trajectory of those changes – whether a 60-year-old’s cells or organs resemble the average lower or higher for their age – can predict how much longer someone is likely to live.

What does biological age tell someone?

Biological age isn’t meant to be just a snapshot of how healthy you are; your doctor already has tests designed for that. Instead, it’s meant to capture the trajectory your body is on, how fast the underlying processes of aging are unfolding, and how many years you likely have left.

“It’s true that, in general, someone healthier is likely to live longer,” said Brunet, the Michele and Timothy Barakett Endowed Professor. “But it’s not completely linked; one person could live to 94 and be in terrible health for the last two decades of their life while another person lives to 94 in superb health until the end.”

Scientists in the field, Brunet said, are still untangling the links between biological age, lifespan (the number of years someone is alive), and healthspan (how long they’re living in reasonably good health).

Wyss-Coray says he thinks of biological clocks as a way to capture someone’s functional health in a more reproducible, quantitative way. He compares it to a measure that the World Health Organization recently dubbed “intrinsic capacity,” which gauges someone’s cognition, mobility, energy, and mood, among other factors.

“Intrinsic capacity asks, What can you do, given your age? How fast can you walk? How much weight can you lift?” Wyss-Coray explained. “It’s really a reflection of how functional you are, given your age. And that’s what we’re trying to capture at a molecular level.”

How scientists measure biological age

There’s no single agreed-upon way to measure biological age. Researchers are coming at the challenge from different angles, each looking at how different aspects of biology change with age, and what those numbers can predict.

Epigenetic clocks. Your DNA accumulates chemical marks called methylation that control which genes are turned on and off in your cells. These marks are part of what is called the epigenome, and the way they change is known as an epigenetic clock. Scientists have shown how the predictable changes in DNA methylation can be used to determine biological age. The earliest epigenetic clocks, the Hannum and Horvath clocks, both debuted in 2013. They were initially designed to capture – as closely as possible – someone’s chronological age with epigenetic data. But an interesting aspect was that epigenetic clocks could also capture how a person’s epigenomic signature deviated from the average, providing a measure of “biological age.”

More recent epigenetic clocks, like a widely used clock called GrimAge, have been designed not to capture a person’s biological age, but something slightly different and perhaps more informative – their mortality or future disease risk.

“At least in a research setting, I’ve found these clocks to be more useful because they’re more focused on capturing the biology of aging that predicts these diseases rather than just how long you’ve been alive,” said David Rehkopf, PhD, professor of epidemiology and population health.

Proteins in the blood. The levels of thousands of proteins throughout your body also change as you age. Some of these can precisely capture the aging of individual organs, rather than the body as a whole. Levels of a protein made only in the brain, for instance, might capture how your brain is aging – which could be at a different speed than your other organs.

“Now that I know more about aging, I’ve learned it’s not just that a few proteins change as you get older; it’s that almost every single protein changes,” Wyss-Coray said.

His lab has datasets tracking how thousands of different proteins collected in routine blood samples change with age; they’re studying which of those proteins come from which organs and what that can tell them about each organ’s health. Many of these proteins are involved in inflammation and the immune system; others are involved in tissue repair and maintenance.

Gene transcription. Some age clocks skip DNA’s chemical modifications and instead look at RNA – the transcripts a cell produces when its genes are actively in use – to get a readout of what’s happening inside cells. The drawback, Brunet said, is that this kind of data can be noisy; gene transcription can change quickly and often.

Telomere length. Telomeres, protective caps at the end of chromosomes, shorten each time a cell divides. Measuring the length of chromosomes is one of the oldest proposed biomarkers of aging. But studies have shown that it may not be the most accurate.

Metabolites. Wearables and some consumer blood panels estimate biological age from vital signs like heart rate and from metabolic markers like cholesterol and blood sugar. These numbers are often the same ones already used by your doctor to capture a snapshot of your health at a routine physical.

How precise they are is still unclear. You could make a clock based on how much gray hair a person has and it would correlate with age. But does that capture anything more broadly about your health or aging trajectory?

“These things often don’t have a lot of strong data on outcomes behind them. I wouldn’t put too much faith in their precision,” Rehkopf said.

Why different tests give different results

If you’ve taken two different biological age tests and gotten two different numbers, it’s not necessarily because one is right and one is wrong. The tests could just be measuring different aspects of your biology.

Imagine, for instance, a 70-year-old marathon runner in optimal cardiovascular health who is in the earliest stages of cognitive decline. Their heart rate, blood pressure, and the epigenomics of their blood cells look younger than 70. But proteins from their brain, circulating into their bloodstream, reveal a picture of disease and older age.

“A lot of the commercial tests out there right now use epigenetics, and they are really just giving you the age of blood cells, because that’s the only DNA they can collect,” Wyss-Coray said. “But they can’t really know how other organs are aging because they don’t have DNA from those organs.”

Each test also has error and noise. Rehkopf estimates roughly three years of error built into many epigenetic clocks – meaning a result showing you’re three years “older” could just as easily mean you’re really three years younger. Some of that noise comes down to which specific blood cells are circulating at what levels when you get your blood drawn; those cell levels can change over the course of just a day.

What influences biological age?

• Decades of research have identified behaviors reliably linked to a longer lifespan.
• Diets rich in vegetables, whole grains, and lean protein, and lower in processed foods, are linked to lower rates of heart disease, diabetes, and other age-related diseases.
• Regular physical activity, including both aerobic exercise and strength training, is associated with a longer lifespan.
• Consistently getting enough quality sleep is linked to better cardiovascular, metabolic, and cognitive health.
• Chronic stress has been tied to a range of negative health outcomes, and people with better stress-coping strategies tend to fare better as they age.
• Strong social ties are one of the more consistent predictors of healthspan and longevity in large population studies.

What’s less established, however, is how reliably any of those behaviors show up on a biological clock test. Researchers are studying which factors influence biological age, whether a clock can move backward after an intervention, and what that says both about the clocks and what it means about the biology of aging.

“There are many factors that influence aging, and they involve many different levels,” Brunet said. “It’s a complex and multifactorial phenomenon.”

Even well-studied interventions can produce confusing, sometimes contradictory signals on these tests. Brunet points to a recent debate over how epigenetic clocks in humans respond to rapamycin, a drug that reliably extends both lifespan and healthspan in animal studies. However, epigenetic clocks recorded a slightly higher biological age in humans on rapamycin – at least at the doses described.

“What that means to me isn’t that rapamycin doesn’t do anything for healthspan and lifespan, but that we don’t understand everything yet about what these clocks are measuring and how we can best use them to tell us the impact of an intervention,” Brunet said.

Rehkopf also notes that most biological age tests are slow to respond to change. Even a real, sustained shift in your behavior and health can take years to show up in an epigenetic blood test. In other words, if you take a test that tells you your biological age, it might not reflect the last few years of your health and habits.

What should you do with your results?

If you’ve already taken one of these tests, the researchers who study biological aging offer this general advice: Don’t expect it to tell you much you didn’t already suspect.

“There’s actually a pretty decent correlation between self-reported health and whether you’re biologically older or younger,” Rehkopf said. “It might not give a lot of new insight for most people.”

But if seeing a number on a test result is important to you, Wyss-Coray believes it can have some value.

“For some people, getting a good result might make them feel better about the choices they’re already making,” he said. “For other people, it might motivate them to make a lifestyle change.”

Most of all, don’t take the result for more than it’s worth.

“People need to understand that it’s more complex than this one number, and that they should not freak out if a clock says they’re older than their calendar age,” Brunet said. “By the same token, if they have a clock indicating they’re younger than their age, that is to be taken with caution, too.”

The bottom line: Clocks are a tool to learn more about aging

For all the uncertainty about any single test, researchers in aging science are excited about the fact that these tests exist. Today, because of their lack of FDA approval, biological age tests are not used clinically by mainstream clinicians and are not covered by typical insurance plans. Instead, consumers can purchase them for several hundred dollars from the companies that produce them.

But while they may not hold immense value for the average consumer right now, they are important tools to address basic questions about aging.

Wyss-Coray, for instance, has spent years following volunteers with monthly blood draws, creating organ-specific biological clocks that give consistent results. Now, he’s studying which interventions move the needle on those results. The goal isn’t to completely reverse all aspects of aging, but to develop a new way for patients and clinicians to precisely track their health.

And if his lab can now show that targeted interventions change organ-specific aging signals, it means the tests can help inform whether a treatment is working in any given organ, months or years before a disease might otherwise show up.