How to Test Your Biological Age — and What the Result Means

This article compares the practical methods of estimating biological age; it does not interpret any individual test result.

The short answer

There is no single, definitive test of biological age. Instead, several different methods each try to estimate how "old" your body appears, and they measure different things. The main options are epigenetic clocks (which read chemical marks on your DNA), blood-biomarker composites such as PhenoAge (which combine routine blood results), functional measures such as grip strength and VO2 max (which capture how your body performs), and telomere tests (which measure the protective caps on chromosomes).

These methods do not agree with one another, and most consumer versions carry more measurement noise than their marketing implies. A biological-age figure is best read as a rough, directional signal rather than a precise verdict. The most reliable insight usually comes not from a single dramatic number but from a measure you can repeat over time, ideally one tied to evidence about real health outcomes. For the underlying science of how DNA-methylation clocks are built, see our explainer on what epigenetic clocks measure.

What "biological age" actually means

Chronological age is simply time since birth. Biological age is an attempt to capture how worn or resilient your body is, on the assumption that two people of the same chronological age can differ markedly in their underlying health. The concept draws on the hallmarks of ageing — the cellular and molecular changes that accumulate over a lifetime.

The catch is that ageing is not one process with one readout. Every test described below is a proxy: it measures something correlated with ageing and converts that into a number expressed in years. Because each proxy captures a different slice of biology, "your biological age" depends heavily on which test you used. Keeping that in mind is the single most useful thing when interpreting any result.

Epigenetic clocks (consumer DNA-methylation kits)

Epigenetic clocks estimate age from DNA methylation — chemical tags that switch genes on and off and shift in patterned ways across life. The first widely used versions were the Horvath multi-tissue clock, built from around 8,000 samples and 353 CpG sites [Horvath 2013], and the Hannum blood clock [Hannum 2013]. Later "second-generation" clocks were trained not on chronological age but on health outcomes: PhenoAge [Levine 2018] and GrimAge, the latter reported to predict time to death and to coronary heart disease more accurately than chronological age [Lu 2019]. A further approach, DunedinPACE, estimates the rate of ageing rather than a fixed age, derived from two decades of within-person decline in the Dunedin birth cohort [Belsky 2022].

These are genuine scientific advances. The practical problem for consumers is reliability. Research found that technical noise alone could produce differences of up to nine years between two replicates of the same sample for six prominent clocks [Higgins-Chen 2022]. In other words, running your spit through the same clock twice on the same day could return answers years apart. The authors developed retrained "principal component" versions that brought most replicate measurements within about 1.5 years of each other — a substantial improvement, but not all consumer kits use these reliable versions, and few disclose which they use.

A second issue is that different clocks disagree. They share little in the way of underlying CpG sites and are often only weakly correlated with each other, which suggests they capture distinct biological processes rather than one unified "age" [Liu 2020]. A kit reporting you are "five years younger" tells you little unless you know which clock produced it and how reproducible that clock is.

Blood-biomarker composites (PhenoAge and similar)

A different route skips DNA entirely and combines results from a standard blood panel. The best-known example, PhenoAge, integrates chronological age with nine routine markers — albumin, creatinine, C-reactive protein, alkaline phosphatase, glucose, lymphocyte percentage, mean corpuscular volume, red cell distribution width and white cell count — into a single age estimate developed using data from more than 9,000 people [Levine 2018].

The appeal is practical. The inputs come from inexpensive, widely available tests, the calculation is transparent, and the components are individually meaningful to a clinician. The same nine numbers that feed the score also flag specific issues such as inflammation or kidney function, which a clock cannot do. The limitation is that these markers fluctuate with short-term factors — a recent infection, acute illness, dehydration or even a heavy training session can move inflammatory and metabolic markers — so a single snapshot may not reflect your stable baseline. Repeating the panel when you are well, and watching the trend, is more informative than one reading. This is a recurring theme across every method here.

Functional measures: grip strength, VO2 max and movement tests

Some of the most robust signals of biological ageing are not laboratory assays at all but simple measures of how your body works.

• Grip strength. In the large international PURE study, each 5 kg reduction in grip strength was associated with a roughly 16% higher risk of death from any cause, and grip strength predicted mortality more strongly than systolic blood pressure [Leong 2015]. It is cheap, quick and measured with a hand dynamometer.
• Gait speed. A pooled analysis of more than 34,000 older adults found that walking speed predicted survival about as well as a combination of age, sex, chronic conditions, smoking, blood pressure and hospitalisation history [Studenski 2011].
• Cardiorespiratory fitness (VO2 max). In a study of over 122,000 people undergoing treadmill testing, higher fitness was associated with lower all-cause mortality with no observed upper limit of benefit; the least-fit group faced several times the risk of the fittest [Mandsager 2018]. Note that lab-measured VO2 max and the estimates from a wrist wearable are not the same thing — see VO2 max: lab vs watch for why the watch figure should be treated cautiously.

The strength of functional measures is that they reflect outcomes people actually care about — mobility, independence, resilience — and many can be tracked at home or in a gym. The weakness is that they capture whole-body function rather than a cellular ageing process, and several (gait speed in particular) were validated mainly in older populations, so they are less discriminating in younger, healthy adults.

Telomere tests

Telomeres are the protective caps at the ends of chromosomes, and they tend to shorten with age, which is why they are sold as a biological-age readout. The evidence here urges particular caution. An international collaborative study found that telomere-length measurements varied substantially between laboratories using the same samples, raising real questions about reproducibility for any single test [Martin-Ruiz 2015]. Consumer qPCR tests in particular have been reported to carry high day-to-day variability.

There is a further interpretive problem. Telomere length and epigenetic-clock estimates have been found to be only independently — not strongly — associated with age and mortality, meaning they are not interchangeable proxies for the same thing [Marioni 2016]. Researchers studying telomere biology have cautioned that direct-to-consumer results can be alarming without being meaningful, for example labelling a healthy person's telomeres as decades "older" than their actual age. On current evidence, telomere length is a useful research tool but a weak basis for a personal biological-age verdict.

How the methods compare

What a consumer test can and cannot tell you

A biological-age test can offer a directional sense of how your overall health markers compare with what is typical for your chronological age, and — used consistently over time — it can hint at whether you are trending in a favourable direction. Functional measures in particular connect to outcomes that matter for healthspan rather than just lifespan.

A consumer test cannot diagnose disease, predict your individual lifespan, or prove that a supplement, diet or programme is "reversing ageing". The clocks and scores were developed and validated at the level of populations, where averages are stable; applying them to one person, on one day, with one kit involves far more uncertainty than a tidy "your age is X" number suggests. Claims that an intervention lowered biological age should be read with the same scepticism you would apply to any health claim — including whether the change exceeds the test's own measurement noise. The same critical-reading habits covered in how to read a clinical trial apply directly to biological-age marketing.

How to interpret a result without over-reacting

A few principles help keep any result in proportion.

• Treat the number as a range, not a point. Given the documented measurement noise, a result a few years above or below your chronological age may carry little signal on its own.
• Favour repeatable, outcome-linked measures. Grip strength, fitness and a standard blood panel are easy to repeat and tied to well-studied outcomes, which makes a trend over months more meaningful than any single exotic test.
• Know which test produced the number. "Five years younger" means different things from a first-generation clock, a mortality-trained clock and a pace-of-ageing measure.
• Watch the direction, not the decimal. Whether a measure is improving, stable or worsening over repeated tests usually matters more than its exact value.
• Do not let a result drive medical decisions on its own. A surprising or worrying figure is a prompt for a conversation with a qualified professional, not a diagnosis.

Biological-age testing is a fast-moving and genuinely interesting field, and the underlying science — particularly the second-generation clocks and pace-of-ageing measures — continues to mature. For consumers today, the honest summary is that these tools can be informative when used carefully and repeatedly, but no current test delivers a single, precise answer to the question "how old am I, really". You can explore the wider evidence base across our longevity coverage.

Related Proco pages

• What epigenetic clocks measure
• The hallmarks of ageing
• Healthspan vs lifespan
• How to read a clinical trial

Sources

If a test result worries you or seems at odds with how you feel, discuss it with a qualified healthcare professional rather than acting on it alone.