Two different ways to count age
Chronological age is simply time since birth. Biological age is an estimate of how worn or resilient the body actually is, based on measurable markers, and two people of the same chronological age can have biological-age estimates that differ substantially. Estimators include epigenetic clocks built from DNA methylation patterns, such as the Horvath clock and the mortality-and-healthspan focused GrimAge, alongside markers like inflammation, organ function, and physical performance.
These clocks are research and increasingly commercial tools, not perfect readouts; different clocks can disagree, and their predictions are statistical rather than individually precise.
Why the distinction is useful
The appeal of biological age is that, unlike chronological age, it may be modifiable: lifestyle factors such as smoking, activity, sleep, and diet are associated with the markers these clocks read. That said, showing a clock moves with an intervention is not the same as proving the intervention extends lifespan, and the field is still validating how meaningful small changes are.
This is general educational information, not medical advice. Direct-to-consumer biological-age tests vary in quality and should be interpreted with appropriate professional context.