1. Biomarkers of Aging Report Position on the Aging Trajectory

Formally, a biomarker of aging is a quantitative parameter that either alone or in a composite predicts biological age of a biological system and ideally its changes in response to health-modulating interventions (11). More specifically, a biomarker of aging is a measurable feature (molecular, cellular, physiological, functional, etc.) that reports where a cell, organ, or organism lies on the given projection of aging trajectory of a reference group/population (biological age) or how fast it is moving along it (aging rate), ideally in a way that predicts future disease or mortality risk and response to interventions. Composite biomarkers integrate many components and are usually more robust. Useful biomarkers are reproducible, interpretable, and responsive in expected directions to interventions and stresses. Because aging is heterogeneous across organs, subsystems, modalities, and timescales, biomarker validity is often purpose-specific, and disagreement among biomarkers can be informative about which aging processes dominate in a given context.