Functional, molecular, and digital measurements of biological age.

The reality of an aging population demands a deeper understanding of aging as a biological process, rather than as a chronological descriptor. Chronological age poorly captures interindividual heterogeneity in physiological and functional decline, disease susceptibility, and mortality risk. In contrast, biological age encompasses deterioration at the molecular, cellular, tissue, organ, functional, and organismal levels and provides insight into why two individuals with the same chronological age exhibit differences in physiological function, disease susceptibility, and mortality risk. While early models of biological age relied on functional markers or composite scores derived largely from longitudinal cohort studies, more recent models integrate molecular profiling with machine learning to ascertain biological aging trajectories. In parallel, new artificial intelligence tools have been applied to various imaging modalities and other forms of complex data to elucidate latent patterns and estimate biological age. In this state-of-the-art Review, we explore historical and modern approaches to estimating biological age and highlight key conceptual, technical, and translational challenges that remain unresolved. As geroscience-guided interventions are incorporated into clinical evaluations, robust and accurate interpretable measures of biological aging are crucial to ascertain treatment effects in clinical trials.