Chronological age vs biological age

“How old are you?” has a precise answer and a fuzzy one. The precise answer is your chronological age, the time since you were born. The fuzzy one, increasingly measured with lab tests, is your biological age: how old your body seems compared with other people’s. They’re related but different, and it helps to know what each can and can’t tell you.

Chronological age: time since birth

Chronological age is simply calendar time: years, months and days since your date of birth. It’s exact, objective and the same for everyone born on the same day. That makes it the measure used for laws, schooling, sport, pensions and most of daily life. It’s also what our age calculator works out.

In psychology and education, “chronological age” has a specific use: a child’s exact age on a test date decides which age norms their scores are compared with. Our chronological age calculator is built for that job.

Biological age: how old your body seems

People of the same chronological age can be in very different health. Biological age tries to capture that difference with a number: if your measurements look like those of a typical 45-year-old, your biological age might be estimated at 45, whatever your birthday says. There’s no single agreed definition. Each method estimates biological age in its own way, so different tests give different numbers.

How biological age is estimated

Epigenetic clocks

Your DNA sequence stays the same through life, but chemical tags on it change. One kind, DNA methylation, changes with age in such a regular way that it can be used as a clock. A landmark 2013 study in Genome Biology introduced a “multi-tissue” epigenetic clock built from methylation at 353 sites in the genome, which estimated age accurately across many tissues and cell types.

Later clocks were trained not just to predict age but to predict health. One, published in 2018 in the journal Aging, combined methylation with a “phenotypic age” derived from routine blood test results, and was better at predicting illness and death than chronological age alone. Others measure the pace of aging rather than a single age: DunedinPACE, published in eLife in 2022, was developed by following a group of people born in Dunedin, New Zealand, in the early 1970s and measuring how quickly their bodies changed over decades.

Blood tests and other biomarkers

Some biological age scores use ordinary clinical measurements, such as blood sugar, kidney and liver function and markers of inflammation, combined into a single estimate. Others use physical measures like grip strength, walking speed or lung function.

Telomeres

Telomeres, the protective caps at the ends of chromosomes, shorten as cells divide, and were once popular as a measure of aging. Telomere length varies a lot between people of the same age, so on its own it’s now considered a weak estimate of biological age.

What affects biological age?

Studies have linked faster epigenetic aging with factors such as smoking, obesity and chronic stress, and slower aging with things like regular physical activity. Most of this research is observational, which means it shows associations rather than proving cause and effect, and it describes averages across large groups rather than predictions for one person. The overlap with general health advice is no coincidence: the habits linked to a younger biological age are largely the ones already known to be good for you.

Should you take a biological age test?

Consumer tests are widely available, usually from a cheek swab, saliva or a blood spot. Before buying one, it’s worth knowing that:

• Different tests use different clocks, so your result depends on which one you take.
• Results can vary between repeated samples and between labs.
• A biological age isn’t a medical diagnosis, and a “good” result doesn’t replace checkups.

If you’re curious, treat the result as one rough data point. For anything health-related, a doctor is the right person to ask.

Biological age in animals

The same idea is used in animals. A 2020 study measured DNA methylation in Labrador retrievers and compared it with humans, producing a formula for “dog years” that is far more realistic than multiplying by seven. Read about it in our guide to dog years and human years, or try the dog age calculator.