An age test tells you the number. This tells you why.

A 2025 review in Epigenomics put numbers on how precise these clocks are. Intraclass correlation runs 0.86 to 0.99 depending on the clock, with coefficients of variation of 2% to 10%. The same DNA sample, split and run twice, has returned biological-age estimates up to 9 years apart. First-generation clocks carry a median absolute error of at least 3.6 years against known chronological age, and the review notes that a difference of a year or two between two readings “may fall within the margin of error.”

That is the case for the markers on this page rather than against them. The drivers below are measured on standard assays with established analytical precision, and each one has interventional human evidence behind the direction it moves. The clock number is the headline. These are the readings that can resolve a change.

The review characterizes clock output as “probabilistic information derived from group-level associations, not mechanistic insight,” and concludes that “applying epigenetic clocks A test that estimates biological age from chemical marks on DNA (DNA methylation). in individual-level decision making can be uninformative and potentially harmful.” No individual-level evidence shows that moving a clock reading improves a health outcome.