7. Biological Age Is a Latent Measure of Position Along the Aging Trajectory

What is biological age? Biological age is an inferred operational state along the aging trajectory, a summary that can be approximated as a scalar but is more faithfully represented as a multidimensional vector of where an organism, organ, or cell lies (10–13). It is not directly observable; it is estimated from measurable features that reflect the aging process. Complex systems have complex organization and temporal properties of the deleteriome (e.g., in cell compartments, cells, tissues, and whole organisms). In this framework, biological age is a projection of the deleteriome onto measurable features—molecular, cellular, physiological, or functional. Practically, biological age can be estimated using biomarkers of aging. A useful biological age measure may track risk-relevant deterioration and respond in expected directions to health-modulating interventions and stressors.