What is biological age? The science of ageing and how to influence it

Two people can be born on the same day and be the same chronological age to the hour, yet one has the arteries, lung function and immune resilience of someone a decade younger, while the other has already crossed into the risk profile of a much older person. That gap is what longevity science is really chasing. Chronological age — the number of times you have travelled around the sun — is fixed and tells you almost nothing modifiable. Biological age is an attempt to measure how worn your body actually is, and unlike your birthday, it appears to be partly within your control.

This article explains what biological age means, what the ageing process actually consists of, how scientists try to measure it, which health systems and blood markers track it, and what the evidence associates with a slower pace of ageing. It is educational, not medical advice — every substantive claim is cited, and the limitations are spelled out as we go. For any decision about your own health, see the medical disclaimer and speak to a clinician.

Chronological age versus biological age

Chronological age is a clock that ticks at the same rate for everyone. Biological age is a clock that ticks at a different rate for each person, and sometimes for each organ within the same person. The reason this distinction matters is simple: measures of biological age have been shown to track disease risk and mortality more closely than chronological age alone (Levine et al., 2018, Aging; Marioni et al., 2015, Genome Biology). The whole point of measuring it is that, where chronological age is a dead end, biological age is a signal you can potentially move.

It helps to separate two ideas that often get blurred. Biological age is an estimate of where you sit on the ageing trajectory right now — effectively, "how old does your body read?". The pace of ageing is a different and arguably more actionable quantity: how fast you are moving along that trajectory — how many biological years you accrue per calendar year. You can be biologically "old for your age" but currently ageing slowly, or biologically young but ageing fast. Modern tools try to capture both.

What ageing actually is: the hallmarks

For most of history, ageing was treated as a vague, inevitable wearing-down. That view has been replaced by something far more specific. In an influential 2013 review, López-Otín and colleagues distilled the biology of ageing into a defined set of hallmarks — interconnected molecular and cellular processes that drive the decline (López-Otín et al., 2013, Cell). A 2023 update expanded the framework to twelve hallmarks (López-Otín et al., 2023, Cell).

The hallmarks include genomic instability (accumulating DNA damage), telomere attrition (the protective caps on chromosomes shortening), epigenetic alterations (changes in how genes are switched on and off), loss of proteostasis (the cell's protein-quality-control system faltering), deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence (cells that stop dividing but linger and emit inflammatory signals), stem-cell exhaustion and altered intercellular communication. The 2023 update added disabled autophagy, chronic inflammation and dysbiosis of the gut microbiome.

The practical message is that ageing is not one thing. It is a small number of measurable, partly modifiable processes acting together — which is exactly why a single "biological age" number can be informative, and also why no single test can capture the whole picture.

How biological age is measured: the family of ageing clocks

There is no one canonical biological-age test. Instead there is a growing family of "clocks", and they do not all measure the same thing. Understanding the differences is the difference between reading your result wisely and over-interpreting it.

First-generation epigenetic clocks

The first epigenetic clocks were built around DNA methylation — chemical tags that sit on DNA and change in patterned ways as we age. In 2013, Steve Horvath showed that the methylation state at a few hundred specific sites could predict chronological age across virtually every human tissue with striking accuracy (Horvath, 2013, Genome Biology). The same year, Hannum and colleagues built a blood-based methylation clock (Hannum et al., 2013, Molecular Cell). These first-generation clocks were trained to predict chronological age, so their real value is in the residual — when your methylation age runs ahead of your birthday, that "age acceleration" tends to track worse outcomes. Prospective cohort analyses, with replication across independent cohorts, found that an older epigenetic age relative to chronological age predicted higher all-cause mortality (Marioni et al., 2015, Genome Biology).

Second-generation clocks: trained on health, not birthdays

The next generation was trained on health outcomes rather than the calendar, which made them more relevant to longevity. PhenoAge, developed by Morgan Levine and colleagues, combines routine clinical-chemistry markers with chronological age into a single "phenotypic age", and its epigenetic version predicts lifespan and healthspan more closely than chronological age does (Levine et al., 2018, Aging). GrimAge went further, incorporating methylation-based surrogates for plasma proteins and smoking history, and proved a strong predictor of time-to-death and time-to-disease (Lu et al., 2019, Aging). If you want the detailed mechanics of PhenoAge specifically — the individual blood markers it uses and how the mortality model works — we cover that in Biological Age Explained: what PhenoAge actually measures.

Pace-of-ageing clocks

A newer approach asks not "how old are you biologically?" but "how fast are you ageing right now?". DunedinPACE, built from a single birth cohort followed for decades, estimates the rate of biological ageing as biological years gained per calendar year — a value around 1.0 means an average pace, below 1.0 slower, above 1.0 faster (Belsky et al., 2022, eLife). Because it measures speed rather than accumulated mileage, this kind of clock is, in principle, well suited to tracking whether an intervention changes the rate of ageing; in a randomised caloric-restriction trial, DunedinPACE did register a slowing (Waziry et al., 2023, Nature Aging).

Blood-chemistry composites

Not every biological-age estimate needs a methylation array. Composite scores built from standard blood panels — markers of metabolism, inflammation, kidney and liver function, and red- and white-blood-cell indices — can also estimate biological age, and they have the advantage of using tests that are cheap, widely available and directly modifiable. This is the layer most accessible to anyone with a recent blood test.

A crucial caveat across the whole family: the clocks correlate, but they do not agree perfectly. Two valid clocks can give the same person somewhat different ages, because they were trained on different data to predict different things. A biological-age number is a research-grade estimate with genuine measurement noise — useful as a trend and a prompt, not a diagnosis.

Which health systems drive biological ageing — and the markers that track them

If the hallmarks are the underlying biology, several everyday, testable health systems are where that biology shows up in numbers you can actually measure and influence.

• Metabolic health. How your body handles glucose is one of the most consistent threads in ageing. Chronically high blood sugar drives vascular damage through a hyperglycaemia-induced overproduction of reactive oxygen species (Brownlee, 2001, Nature), and markers such as HbA1c and fasting glucose give a window onto it — see HbA1c and the link between blood sugar and ageing.
• Inflammation. A low-grade, chronic inflammatory state — termed "inflammaging" — is a significant risk factor for morbidity and mortality in later life, because many age-related diseases share an inflammatory component (Franceschi and Campisi, 2014, Journals of Gerontology Series A), and it was added explicitly to the 2023 hallmarks. hs-CRP is a widely available blood-test window onto it: hs-CRP, inflammation and ageing.
• Cardiovascular and lipid health. Arterial disease tracks the number of atherogenic particles in circulation rather than cholesterol mass alone, and apolipoprotein B counts those particles directly, which is why it is increasingly favoured as a risk marker (Sniderman et al., 2019, JAMA Cardiology): why ApoB can beat LDL cholesterol for heart risk.
• Cardiorespiratory fitness. How efficiently your body uses oxygen is closely tied to long-term survival: higher cardiorespiratory fitness is associated with lower mortality, with no observed upper limit of benefit in the data (Mandsager et al., 2018, JAMA Network Open): VO2 max as a longevity biomarker.
• Organ-system reserve. Kidney function, liver enzymes and the red- and white-cell indices that appear on a routine blood count all feed into composite biological-age scores, because they reflect the cumulative state of organ systems rather than any single disease.

No one of these is biological age. Together they form the measurable surface of the deeper hallmarks — and, importantly, most of them respond to the same handful of lifestyle levers.

What the evidence links to a slower pace of ageing

This is where the field becomes genuinely encouraging, with the important honest caveat that most human evidence is observational and shows association rather than proof of cause. With that stated plainly:

• Physical activity and fitness are consistently associated with longer life. As noted above, higher measured cardiorespiratory fitness is associated with lower long-term mortality, with no observed upper limit of benefit (Mandsager et al., 2018, JAMA Network Open). Movement recurs throughout the longevity literature as a broadly beneficial factor.
• Diet quality appears in the same epigenetic-clock analyses: healthier lifestyle and dietary factors were associated with lower epigenetic age acceleration, while higher BMI and metabolic-syndrome measures were associated with greater acceleration (Quach et al., 2017, Aging). In a randomised controlled trial, sustained caloric restriction in healthy adults slowed the pace of ageing as measured by DunedinPACE (Waziry et al., 2023, Nature Aging) — among the first randomised human evidence that an intervention can shift a pace-of-ageing clock.
• Sleep, smoking and alcohol are among the everyday exposures most consistently linked to cardiometabolic health. Smoking in particular is one of the components GrimAge explicitly incorporates, through a methylation-based surrogate for smoking exposure, and it is a powerful predictor of mortality and disease in that model (Lu et al., 2019, Aging).
• Investigational pharmacology. Drugs such as metformin and the mTOR inhibitor rapamycin are being studied as potential geroprotectors: metformin is the basis of the proposed TAME trial (Barzilai et al., 2016, Cell Metabolism), and a rapamycin analogue improved a marker of immune ageing in a randomised trial in older adults (Mannick et al., 2014, Science Translational Medicine). These remain investigational for ageing specifically — they are not established anti-ageing treatments, and any use sits firmly in the domain of a clinician, not a blog.

The throughline is that the levers with the best evidence are unglamorous and familiar — regular movement, cardiorespiratory fitness, a mostly whole-food diet, sufficient sleep, and not smoking. The novelty is not the list — it is that we can increasingly measure whether these things are working.

How to read a biological-age result sensibly

A few principles keep a biological-age number useful rather than misleading. Treat the trend over time as more informative than any single reading — measurement noise means one result is a snapshot, not a verdict. Expect different clocks to disagree somewhat; that is a property of the methods, not an error. Remember these are research-grade estimates, not medical diagnoses — an older-than-expected result is a prompt to look at the modifiable systems behind it, not a cause for alarm. And focus your attention where the biology is actually movable: the metabolic, inflammatory, cardiovascular and fitness markers above are where lifestyle changes show up first.

The bottom line

Biological age reframes ageing from something that simply happens to you into something with measurable drivers, several of which respond to how you live. The hallmarks of ageing give us the underlying biology; the family of ageing clocks gives us imperfect but improving ways to read it; and a small set of well-evidenced habits gives us levers that, in the best available studies, are associated with — and in at least one randomised trial appear to slow — the pace of ageing. Used as a trend and a prompt rather than a diagnosis, a biological-age estimate is one of the more motivating numbers in modern health.

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This article is for education only and is not medical advice. See our medical disclaimer.