Epigenetic clocks

Epigenetic clocks are biological aging biomarkers derived from patterns of DNA methylation across the genome. The core discovery, made independently by Steve Horvath and Gregory Hannum in 2013, is that the methylation state of a specific set of CpG sites across the genome predicts chronological age with striking accuracy — and, more importantly, that deviation from this predicted age (biological age relative to chronological age) predicts mortality, disease risk, and the effects of interventions such as caloric restriction.

The Horvath clock, trained on methylation data from many tissue types, predicts age with a median error of approximately 3.6 years across a wide range of human tissues. Subsequent clocks — Hannum, PhenoAge, GrimAge — have been trained on different outcomes (chronological age, physiological aging markers, mortality) and make somewhat different predictions. What all share is the identification of methylation drift as a systematic, calibrated process whose deviation from expectation carries health information.

Epigenetic clocks have become central instruments in aging research and longevity interventions, including the growing field of biological age reversal that uses methylation clock readouts as outcome measures for interventions ranging from dietary protocols to partial cellular reprogramming. Whether clock age measures the cause of aging or merely a correlated readout of underlying aging processes remains unresolved — a distinction with significant consequences for whether clock reversal constitutes actual rejuvenation or merely cosmetic molecular bookkeeping.