Epigenetic clock

An epigenetic clock is a statistical model that reads DNA methylation from a blood or saliva sample and returns an estimated age or rate of aging. Dozens exist, they disagree with one another, and they are the engine behind almost every consumer biological age test.

They differ in what they were trained to predict. The earliest were built to estimate chronological age. Later ones, including those known as PhenoAge and GrimAge, were trained against health measures or time to death, which makes them better at what people actually want to know and worse at the thing the word “age” implies. A newer family estimates the pace of aging rather than a level.

Reliability has been the field’s persistent problem, and it has two halves that are often confused. Technical reliability asks whether the same sample gives the same answer twice. A 2022 paper in Nature Aging reported that technical noise alone produced deviations of up to nine years between replicates for six prominent clocks, and introduced a principal-component method that brought most replicates within 1.5 years.

Biological reliability asks something harder: whether the same person gives the same answer twice. A 2026 study in Aging Cell evaluated 18 methylation-based aging biomarkers and found that while most were technically reproducible, their stability across repeated samples taken within short intervals — before and after meals, under acute stress, across different exposures — was substantially lower, with most showing only low to moderate stability. The authors noted that technical reproducibility did not predict biological reliability.

That last finding is the one that matters for anyone reading a test result. A clock can be perfectly consistent about a tube of blood and still give a different answer about the person it came from, depending on the day.