Epigenetic Clock (DNA Methylation Age)

An epigenetic clock is a statistical predictor that estimates age from patterns of DNA methylation, the chemical marks on the genome that change in a regular way over the life course. The most influential is Steve Horvath's 2013 multi-tissue clock, which predicts chronological age from methylation levels at 353 specific CpG sites using a penalized regression model. Methylation is measured as a beta-value between zero and one at each site, representing the fraction of cells in which that site is methylated, and the clock combines a weighted set of these values into a predicted DNA methylation age, or DNAm age. Remarkably, Horvath's clock works across many tissues and cell types from the same individual, suggesting it captures a fundamental aging process rather than a tissue-specific quirk. The difference between predicted DNAm age and actual chronological age, known as epigenetic age acceleration, serves as a biomarker of biological aging. Age acceleration predicts mortality and a range of age-related conditions, which has made epigenetic clocks central to modern aging research.