Epigenetics is the study of heritable changes in gene expression that don't involve changes to the DNA sequence. Instead, they involve methylation patterns, histone modifications, and non-coding RNA.
Mechanism: Environmental inputs (diet, stress, sleep, toxin exposure, exercise) alter which genes are expressed. Some changes are reversible; some persist for generations. Aging is measurable epigenetically (Horvath clock, GrimAge).
Use case: Epigenetic age clocks (Horvath, GrimAge, DunedinPACE) estimate biological age from blood. Lifestyle interventions move them measurably over months.
Caveats: Commercial epigenetic-age tests vary in reliability. Use them for trends, not single readings.