Reading age from DNA methylation
DNA methylation is a chemical tag, mostly added at CpG sites, that helps regulate whether genes are switched on or off without changing the DNA sequence itself. Across the genome, methylation at certain sites changes in predictable ways with age. Epigenetic clocks exploit this: by measuring methylation at a selected panel of CpG sites and weighting them statistically, models such as the Horvath and Hannum clocks estimate biological age from a tissue or blood sample.
The first-generation clocks were trained to predict chronological age accurately; later clocks were tuned toward health and mortality outcomes, and the different clocks do not always agree.
Strengths and limits of the method
Methylation clocks are among the more accurate molecular age predictors available and are widely used in aging research, including to test whether interventions appear to slow the clock. Their limits matter too: they are statistical correlations, results depend on the tissue and assay, and a clock reading shifting does not by itself prove a change in real biological aging or lifespan.
This is general educational information, not medical advice. Consumer methylation-age tests vary in rigor and should be interpreted with appropriate professional context.