What ageing clocks actually measure, reconsidered
Epigenetic clocks promise to tell you how old your body really is. They read chemical tags on your DNA and translate them into a biological age.
Epigenetic clocks estimate biological age from DNA methylation: small chemical groups that attach to DNA and influence which genes are active. As we age, these tags shift in a predictable pattern. That gradual divergence is called epigenetic drift, the increasing variation and change in DNA methylation that accumulates over a lifetime.
An analysis published in Nature Aging on 24 June 2026 examines the relationship between epigenetic drift and the clocks built from it. The authors trace how the concept of drift has evolved and how clocks have grown increasingly precise at quantifying it. But they raise a central concern: we still do not fully understand what the measured signal means biologically.
Correlation without a mechanism
The researchers argue that the biological processes driving drift, and the consequences of that drift for cell function and health, remain poorly characterised. A higher clock age correlates with more disease and shorter lifespan in observational data, but correlation is not causation.
What clocks can and cannot do
The analysis puts epigenetic clocks in perspective. They are valuable tools for population studies and group comparisons. But they are not yet direct readouts of the mechanisms that drive ageing. The authors call for more fundamental research into what causes epigenetic drift at the cellular level, and how it relates to tissue and organ function. Only then can clocks evolve from descriptive measures into instruments that also guide therapeutic decisions.