A decade ago, measuring your biological age meant a laboratory, a research grant, and a collaborator willing to run your saliva through a methylation array. Today you can order a kit online, spit into a tube, and receive a number that claims to tell you how fast you are ageing. Meanwhile, a parallel approach skips the methylation step entirely and estimates biological age from routine blood chemistry — the kind of panel a GP might order. Both families of tests predict mortality in large cohorts. Neither is perfect. And the consumer marketing around both often promises far more certainty than the science delivers.
This article compares the two main approaches — epigenetic (DNA methylation) clocks and blood-based composite clocks like PhenoAge — honestly. What each measures, how well it predicts outcomes, what the noise looks like, and what the honest gaps are. Evidence rating: STRONG that biological age measures (both families) predict mortality and morbidity in population studies; EMERGING for their use as a personal health-tracking tool, because no randomised trial has yet shown that acting on a biological age score extends an individual's lifespan.
Two families, one question
Both approaches try to answer the same question: is your body ageing faster or slower than expected for your calendar age? They get there by very different routes.
Epigenetic clocks measure DNA methylation — chemical tags (methyl groups) attached to specific sites on your genome. These tags change in patterned ways as you age, and by measuring thousands of them at once, a statistical model can estimate your biological age. The pioneer was Steve Horvath's 2013 multi-tissue clock, trained on over 8,000 samples across 51 tissue and cell types, which predicted chronological age with a median error of around 3.6 years. That same year, Hannum and colleagues built a separate blood-based methylation clock from 656 individuals, confirming that genome-wide methylation patterns shift quantitatively with age.
Blood-based composite clocks take a different input entirely: standard clinical blood biomarkers — albumin, creatinine, C-reactive protein (CRP), glucose, white blood cell count, lymphocyte percentage, mean cell volume, red cell distribution width, and alkaline phosphatase. The best known is Phenotypic Age (PhenoAge), developed by Morgan Levine and colleagues in 2018. They used a penalised regression model to narrow 42 candidate biomarkers down to nine, plus chronological age, that together predicted mortality in the US National Health and Nutrition Examination Survey (NHANES III) cohort. The resulting composite — and this is important — was originally a clinical-chemistry score, not a methylation measure. Levine's team then also trained a DNA methylation estimator of Phenotypic Age (DNAm PhenoAge, 513 CpG sites), creating a bridge between the two worlds. But the blood-chemistry version of PhenoAge stands on its own as a biological age estimator, without any methylation data at all.
The generational leap: first-gen vs second-gen clocks
Not all epigenetic clocks are created equal, and the distinction matters for anyone considering a test.
First-generation clocks (Horvath 2013, Hannum 2013) were trained to predict chronological age from methylation patterns. They do this remarkably well — but a clock optimised to guess your passport age is not the same as one optimised to predict your health trajectory. If your methylation pattern looks "old for your age," a first-gen clock flags it, but the deviation may or may not be linked to disease risk.
Second-generation clocks shifted the target. GrimAge (Lu et al., 2019) was trained on surrogate markers of smoking, inflammation, and plasma proteins linked to mortality, and it strongly predicted time-to-death in the Framingham Heart Study offspring cohort, with highly significant associations with coronary heart disease and measures of healthspan. DunedinPACE (Belsky et al., 2022) went further still: rather than estimating how old you are, it estimates how fast you are ageing right now — a rate, not a snapshot. It was built from 20 years of longitudinal data tracking physical and cognitive decline in the Dunedin birth cohort, then condensed into a single methylation readout. DunedinPACE predicted morbidity, disability, and mortality, and added predictive power beyond GrimAge.
The practical upshot: if you see a consumer test advertising a "DNA age" or "epigenetic age," it matters enormously which clock it uses. A first-generation clock trained on chronological age is a weaker health predictor than a second-generation clock trained on mortality or pace of ageing.
Blood-based clocks: cheaper, retestable, and surprisingly competitive
The blood-chemistry approach has a structural advantage that is easy to overlook: you already have the data, or can get it from a routine blood draw. PhenoAge's nine biomarkers are standard clinical analytes available in most countries, from most laboratories, at a fraction of the cost of methylation profiling.
In 2021, Kwon and Belsky published an open-source toolkit (BioAge) that implements PhenoAge and two related blood-chemistry biological age algorithms — Klemera-Doubal method (KDM) biological age and homeostatic dysregulation — from customisable sets of blood biomarkers. The toolkit makes it straightforward for researchers to compute biological age from routine blood panels, and the authors demonstrated that these blood-chemistry measures capture meaningful variation in ageing across observational cohorts.
This matters because the blood-based approach does not require methylation profiling. The nine PhenoAge biomarkers are standard clinical analytes — the kind of markers available from routine blood work. While DNA methylation clocks capture tissue-level epigenetic changes that blood chemistry cannot, the practical accessibility and retestability of blood-based measures is a genuine advantage for tracking change over time.
Blood-based clocks also have a practical edge for tracking change over time. Because a standard blood panel can be repeated every few months at modest cost, you can build a personal trend — a trajectory, not a single reading. Epigenetic tests, typically running to several hundred pounds per sample, are harder to repeat frequently enough to separate signal from noise.
The noise problem
No biological age test, regardless of family, delivers a perfectly stable, precise number — and the consumer marketing rarely makes this clear.
For epigenetic clocks, measurement variability arises from multiple sources: the methylation assay platform, sample handling, batch effects, and the choice of clock algorithm. Because first-generation clocks were trained to predict chronological age rather than health outcomes, a change in score between two time points is harder to interpret — it may reflect genuine biological change, or it may reflect technical variation in the assay. Second-generation clocks (GrimAge, DunedinPACE), trained on mortality and pace of ageing respectively, are designed to capture health-relevant signal, but measurement uncertainty is inherent in any methylation-based assay.
Blood-based clocks carry their own noise. CRP is one of the nine PhenoAge biomarkers, and because CRP is an acute-phase reactant, any infection, injury, or transient inflammatory episode can raise it sharply — potentially shifting the PhenoAge composite in ways that do not reflect long-term ageing. Creatinine, another PhenoAge input, is influenced by muscle mass and hydration. These are well-understood sources of biological variability in the analytes that compose the score, and they are why a single blood-based reading should not be over-interpreted.
The honest conclusion: both families of tests are population-level statistical tools that predict group-level outcomes powerfully, but carry meaningful noise at the individual level. A single reading is a starting point, not a verdict. A trend across multiple readings — ideally three or more, spaced months apart — is far more informative.
What the clocks can — and cannot — tell you
It is worth being explicit about the gap between what biological age tests do in research and what consumers are often led to believe.
What they do well (STRONG evidence):
- In large cohorts, both epigenetic and blood-based biological age measures are associated with all-cause mortality and cardiovascular disease, often more strongly than chronological age alone. Second-generation epigenetic clocks also associate with cancer incidence and broader healthspan metrics.
- Second-generation clocks (GrimAge, DunedinPACE) predict these outcomes more strongly than first-generation clocks.
- Blood-chemistry-derived measures (PhenoAge, KDM) capture meaningful variation in biological ageing using routine, accessible blood analytes.
What they cannot yet do (the honest gap):
- As of mid-2026, no randomised controlled trial has shown that reducing your biological age score — by any method — extends your life. The clocks are associated with outcomes at the population level; they have not been validated as surrogate endpoints for individual interventions.
- A single test result carries enough measurement noise that small differences (a year or two of "acceleration") may be artefactual, especially with first-generation clocks trained on chronological age rather than health outcomes.
- Neither family diagnoses any disease, and neither should be used to make clinical decisions without a qualified healthcare professional.
These are correlates of ageing, not causes of it. They are useful — genuinely useful — as one input among many, especially when tracked over time. They are not crystal balls.
How Omniwo helps you measure this
Omniwo's BioAge test uses a blood-based, PhenoAge-style composite clock. From an at-home finger-prick sample, it calculates a biological age score from validated clinical biomarkers — including hs-CRP, the inflammatory marker that features in both PhenoAge and the wider ageing-research literature as a signal of chronic low-grade inflammation.
What makes this approach practical for personal tracking is exactly the advantage the research literature highlights: it is accessible, affordable, and retestable. Rather than a single expensive epigenetic snapshot, you can establish a baseline, change an input (exercise, diet, sleep), and re-test in a few months to see whether your trend is moving in the right direction.
Omniwo does not run DNA methylation or epigenetic clock analysis — and for personal wellness tracking, a retestable blood-based approach has a distinct practical advantage. The PhenoAge composite draws on routine clinical analytes — the same biomarkers that researchers use to study biological ageing in large cohorts. The ability to retest affordably means you build a personal trajectory over time rather than relying on a single data point. Pair the blood work with wearable data from Oura, WHOOP, Strava, Polar, or Apple Health — tracking sleep, HRV, and activity alongside your biomarkers — to build a fuller, multi-layered picture of how you are ageing.
A blood-based BioAge score is a complement to an epigenetic clock, not a replacement. If you want both, start with the accessible, retestable foundation and add a methylation test when it makes sense for you.
Omniwo's tests and content are for wellness and educational insight. They are not a medical device, do not diagnose, treat, cure or prevent any disease, and do not replace advice from a qualified healthcare professional.
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Lu AT, Quach A, Wilson JG, et al. DNA methylation GrimAge strongly predicts lifespan and healthspan. Aging (Albany NY). 2019;11(2):303–327. doi:10.18632/aging.101684 (PMID: 30669119)
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Belsky DW, Caspi A, Corcoran DL, et al. DunedinPACE, a DNA methylation biomarker of the pace of aging. eLife. 2022;11:e73420. doi:10.7554/eLife.73420 (PMID: 35029144)
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Kwon D, Belsky DW. A toolkit for quantification of biological age from blood chemistry and organ function test data: BioAge. Geroscience. 2021;43(6):2795–2808. doi:10.1007/s11357-021-00480-5 (PMID: 34725754)
This article is educational and not medical advice. See our medical disclaimer.