If you’ve ever been to a high school reunion, you know that some people seem to age faster than others. Twenty-five years after graduation, one classmate can appear a decade younger than the rest, another a decade older.

“People know that intuitively,” said Dr Nir Barzilai, director of the Institute for Aging Research at the Albert Einstein College of Medicine, New York, US, “but they don’t understand that it’s a biology that we’re trying to discover.”

Scientists are working to quantify this phenomenon and put a number to a person’s “biological age” by looking at their cellular health instead of how many years they’ve been alive. Some of these measurements are now marketed as direct-to-consumer blood tests. But before you shell out hundreds of dollars to find out how old you really are, make sure you know what you’re paying for. Experts caution that while these tests are interesting in theory, and could be valuable research tools, they aren’t ready for prime-time.

How do you measure biological age? Researchers define biological age as “the accumulation of damage we can measure in our body,” said Dr Andrea Britta Maier, co-director of the Centre for Healthy Longevity at the National University of Singapore. That damage comes from natural ageing, as well as from our environment and behaviours.

The concept is often attributed to British physician-scientist Dr Alex Comfort who published a 1969 paper on the idea. However, for decades, scientists didn’t know how they might measure someone’s biolo-
gical age.

A major advance came in 2013 when Steve Horvath, a professor of human genetics and biostatistics at the University of California, Los Angeles, US, proposed using a “clock” based on the emerging field of epigenetics. Over the course of our lives, our DNA accumulates molecular changes that turn on and off various genes. Horvath analysed these changes in thousands of people and developed an algorithm to determine how they correlate with age.

These changes happen naturally as we get older, said Jesse Poganik, an instructor at Harvard Medical School, US, who researches biological ageing; they can also be sped up by behaviours that affect health, such as smoking and excessive alcohol consumption. As a result, estimates of biological age have been shown to be associated with things like life expectancy and health, he said.

Several companies now sell tests for around $300 that use this technology to calculate your biological age by analysing your blood or saliva and comparing changes in your epigenome to population averages.

But experts caution that epigenetic clocks can’t actually tell you much about your own health. That’s because they were designed to assess large groups of people, not individuals. Consequently, their results can be unreliable.

At a recent conference where Horvath spoke about the topic, an audience member said he had taken two different tests and received two different ages, 10 years apart. Horvath said that the man should have saved his money.

“I think you could say the best of them are not completely useless,” said Daniel Belsky, an associate professor of epidemiology at Columbia University, US, who developed an epigenetic clock himself. “But these are not tried and tested clinical tools yet, so they’re more for the curious.”

Another problem with the tests is that it’s unclear what to do with the results. Scientists don’t know how to reverse someone’s biological age — or whether that’s even possible.

In part, that’s why the epigenetic clocks were developed in the first place. Researchers hope to use them in clinical trials for anti-aging interventions to measure potential changes in the life spans of hundreds or thousands of people at a time.

None of this has stopped companies from selling these tests alongside personalised health and lifestyle recommendations, in addition to supplements they say will roll back an individual’s biological age.

Epigenetic clocks aren’t the only products on the market promising to measure biological age. Some companies offer a panel of conventional blood tests you might receive at the doctor’s office, such as cholesterol or hemoglobin A1C, a marker for diabetes. They say that because many of these numbers increase as we get older, they can be used as a proxy for a person’s biological age. For example, if you’re 45 but your cholesterol levels look more like the average 50-year-old’s, the test results might say your biological age is older than your 45 years.

Whether blood marker tests actually track biological age as opposed to general health is up for debate. But an advantage of this kind of test is that it measures factors that can be modified; we know how to lower blood sugar levels through medication and lifestyle changes, for example. In contrast, epigenetic age is more of a black box.

“Expanding access and using more frequent testing to optimise health seems fairly reasonable to me,” Poganik said via email. But, he added, “any claims of accurate, individual-level determination of biological age should be approached with caution”.

NYTNS