Biological age: what it is and how it's measured

Biological age is an estimate of how old your body behaves, compared with other people. Your chronological age is the number of years since you were born. Two 50-year-olds can have very different hearts, muscles and blood, and biological age tries to put a number on that difference.

There's no single test for it. Every method picks some measurements, compares them with people of known ages and outcomes, and turns the result into years. So the number depends on what was measured and which reference group it was compared with.

Biological age vs chronological age

Chronological age is exact. Biological age is a model. If yours is lower than your real age, your measurements look like those of a younger person, on average. If it's higher, they look older.

The useful part is what the gap predicts. The better methods are built so that a higher biological age goes with a higher risk of disease and death in large studies. That's a statement about groups. It can't tell you how long you personally will live.

How biological age is measured

A 2017 review from the Karolinska Institute grouped the methods into six kinds: epigenetic clocks, telomere length, gene activity, proteins, metabolites and composite scores built from routine measurements. It found epigenetic clocks the most promising, and telomere length weaker.

Epigenetic clocks read chemical tags on your DNA, called methylation, from a blood or saliva sample. The first well-known one, published by Steve Horvath in 2013, was built from 8,000 samples across 51 tissues and cell types and uses 353 sites on the genome. It was trained to predict chronological age, which made it accurate at guessing age but less tied to health.

Later clocks were trained on health instead. PhenoAge, from 2018, was built from nine routine blood markers plus age, then translated into DNA methylation. It predicted death, cancer, heart disease and physical decline better than earlier clocks. GrimAge, from 2019, uses methylation stand-ins for seven blood proteins and for smoking history, and was the strongest predictor of time to death among the clocks its authors compared.

DunedinPACE works differently. Instead of an age, it gives a speed, in years of biological aging per calendar year. It was built from people in New Zealand born in 1972 and 1973, measured on 19 markers of organ health four times over two decades. A value of 1 means average speed.

Blood-based ages skip the DNA step. The original PhenoAge formula needs only a standard blood panel: albumin, creatinine, glucose, C-reactive protein, white blood cell count and four more markers. That's what Blood age uses.

Fitness-based ages use how your body performs. Cardio fitness alone is one of the strongest predictors there is. In 122,007 adults who took a treadmill test, the least fit quarter were about five times as likely to die over the following years as the fittest 2%.

How accurate are biological age tests?

Less precise than the decimal places suggest. In 2022, Yale researchers ran the same samples through six popular epigenetic clocks twice and found technical noise alone moved results by up to 9 years. Their fix brought most repeat results within 1.5 years, but not every commercial test uses it.

Different methods also disagree with each other, because they measure different things. A DNA clock, a blood panel and a treadmill test can give three different ages for the same person. None of them is the true one.

So treat any single number as a rough reading. Repeat measurements with the same method, months apart, tell you more than one result does, and a change of a year or two is often within noise.

What a home estimate can and can't tell you

You can estimate a fitness-based age at home with no lab. Cardio fitness can come from a watch, a run or a few questions. Strength, balance, sleep, activity and waist size fill in the rest. These are things you can change, and they each predict how people age.

That's the approach behind Ageon. It compares your results with published reference ranges for your age and sex. Ageon Age is the age at which the inputs that change most with age, like VO2 max, push-ups and balance, would be average. It's an estimate, not a measurement of biological aging, and a lower number doesn't prove your body is younger. The method page lists every table and weight.

You can try the same idea in two minutes with the free biological age calculator.

Which method should you use?

If you want a research-grade number, an epigenetic test with a published, health-trained clock such as PhenoAge, GrimAge or DunedinPACE is the closest thing, but expect noise and a price. If you already get routine blood work, a blood-based age costs nothing extra. If you want something you can measure often and act on, fitness tests are the cheapest and most practical, and the habits that move them are well studied.

Whatever you use, compare yourself with yourself over time, and talk to your doctor about any result that worries you.