27. What does validation mean for an aging biomarker?

Validation is not just correlation with age. A biomarker intended for testing interventions should be reliable and reproducible, predict meaningful outcomes, respond appropriately to known damage inducers, lifespan-extending and lifespan-shortening perturbations, and preferably map to interpretable biology. A useful working standard is as follows: if a marker changes, we should be able to predict what should happen to function and risk, and be surprised when it does not.