How reliable are epigenetic clocks for measuring your biological age?

Epigenetic clocks measure biological age by looking at DNA methylation: chemical tags on the DNA that change in predictable locations as an organism grows older. The accuracy is impressive: a pan-mammalian clock, tested across 185 species and 59 tissue types, achieves a correlation of more than 0.96 between the measured clock age and the actual age^1,2. That is a strong measurement result. The technique does perform less well in certain cell types, such as fibroblasts, which shows that the measurement is not equally reliable in every biological material.

The measurement also says something about the future. Certain clocks, most notably GrimAge, predict mortality and life expectancy better than calendar age alone^3,2. The so-called 'skin & blood clock' is associated with lifespan and age-related conditions. An important caveat: these are associations. The fact that GrimAge predicts someone's risk of death does not mean that the clock is measuring the cause of ageing. The causal mechanism is unknown.

An interesting research question is whether the clock reading can also be changed. The small TRIIM pilot study (9 participants) tested a combination of growth hormone, DHEA and metformin and found that the epigenetic clock age was on average 1.5 years lower than at the start after one year of treatment; the GrimAge difference persisted six months after stopping³. This is striking, but the study was very small, had no control group, and the authors had financial interests in the outcome. On the basis of this study alone, no conclusion can be drawn about whether the treatment actually slows ageing.

DNA methylation is not the only way to measure biological age. A transcriptome-based clock, BiT age, approaches the theoretical accuracy limit and also works in organisms that have no DNA methylation at all, such as the roundworm C. elegans⁴. The fact that different biological processes yield the same clock reading suggests that ageing runs along multiple measurable pathways, and that no single clock tells the complete story.

The fact that the same CpG sites (specific locations on the DNA) change consistently with age across almost all mammalian species suggests that ageing is deeply evolutionarily embedded and interwoven with how organisms develop¹. This gives epigenetic clocks scientific weight as a research instrument. At the same time, it makes clear that 'turning back the biological clock' is not a simple intervention: it touches on processes that have been broadly conserved throughout evolution.