Do epigenetic clocks vary so much that your result is just chance?
Epigenetic clocks can diverge substantially for the same purpose, so your result does genuinely depend on which clock is used. For personal clinical decisions they are not yet reliable enough; for broad lifestyle questions they appear more consistent.
Epigenetic clocks all measure DNA methylation, but they are not copies of one another. This is most clearly illustrated by comparing two clocks that both claim to measure stem-cell division: one did so well (correlation of 0.92 with direct measurements), while the other was almost useless for that purpose (correlation of 0.30). Two clocks, the same label on the box, but completely different outcomes.
When it comes to mortality risk, the choice genuinely matters. Of a range of clocks tested in nearly two thousand people, only GrimAge and GrimAge2 reliably and proportionally predicted the probability of death from all causes, cancer, or heart disease. The other clocks examined scored poorly or not at all on that point. If you want to know whether a clock is relevant for mortality, the choice of model is therefore crucial.
There is, however, one situation in which clocks do agree: population research on lifestyle. Heavy drinking, smoking, and a high body weight showed a comparable pattern across three countries, regardless of which clock the researchers used. That offers some reassurance for anyone who wants to know whether their lifestyle leaves measurable traces in their DNA.
There are also technical limits. A heavily simplified clock with only eight measurement points estimated age by nearly six years on average, and three of those eight points barely correlated with age at all. Cheaper or more compact variants therefore produce a rougher estimate.
The common thread running through several independent evaluations is that no universal standard yet exists, that most clocks require further validation, and that no clock is yet suitable for use in everyday clinical practice. Large-scale genetic research also shows that four widely used clocks partly share the same genetic variants associated with high blood pressure, but that those variants sometimes push the clocks in opposite directions. Biologically, they are demonstrably different, even if they all go by the name 'epigenetic clock'.
This answer draws on three broad review articles and five other studies, including a large genetic study and a study involving nearly two thousand people. The evidence on differences between clocks is reasonably solid; the evidence on when a clock is clinically useful is still thin, because real-world validation studies in practice are lacking.
How reliable are epigenetic clocks for measuring your biological age?
Epigenetic clocks are promising, reproducible markers for biological age, but they have methodological limitations that reduce their reliability outside the laboratory setting. Use them as an indication, not as a definitive statement about your own ageing.
What does an epigenetic clock actually measure?
An epigenetic clock reads chemical tags on your DNA and translates them into a biological age estimate. The further that outcome exceeds your calendar age, the greater the risk of age-related conditions, although it has not been proven that the clock measures the cause of those conditions.
Can an epigenetic age test really tell you how old your body is?
An epigenetic age test provides a meaningful indication of biological ageing, but is not yet a definitive verdict on your health. Use the result as a starting point for a conversation with your doctor, not as a fixed judgement.
Can you influence your epigenetic age through diet?
Multiple studies show that a healthy dietary pattern can favourably influence your epigenetic age, although these are surrogate measurements and results from smaller studies are sometimes clouded by conflicts of interest. A Mediterranean or plant-based diet rich in vegetables, nuts and vegetable oils has the most supporting evidence.
Does your biological age change differently at night than during the day?
The night is biologically not a pause but an active recovery phase, and that phase deteriorates noticeably with age. Those who protect their sleep rhythm, with fixed times, limited screen use before bedtime and a longer nightly eating break, preserve that recovery function for longer.
How can I measure my biological age?
Epigenetic clocks, in particular GrimAge and PhenoAge, currently offer the best estimate of your biological age, but they are not yet ready for straightforward home use and require expert interpretation.