Biological aging clocks as biomarkers of human aging: Biological basis, methodological design, and epidemiological implications

Biological age (BA) has been proposed as a complementary construct to chronological age (CA) for quantifying interindividual heterogeneity in aging trajectories. Biological aging clocks (BACs) integrate molecular, clinical and multi-omics biomarkers to estimate aging-related phenotypes beyond CA. This narrative review critically examines the biological foundations, statistical methodologies, interpretative challenges and translational applications of BACs. We discuss the mechanistic basis of BACs development within the frameworks of the hallmarks and domains of aging, emphasizing the roles of age-related methylome remodeling, immunosenescence, and chronic low-grade inflammation. BACs are classified into three major generations according to their training objectives: first-generation clocks optimized to predict CA, second-generation clocks designed to estimate morbidity and mortality risk, and third-generation clocks developed to quantify the pace of aging from longitudinal biomarker changes. We also review multi-omics and artificial intelligence-based approaches that aim to capture the multidimensional nature of aging. Important limitations remain regarding biological specificity, causal interpretation, reverse causation, confounding, generalizability and clinical applicability of BACs. Current evidence suggests that BACs represent distinct operationalizations of biological aging instead of interchangeable measures of a single construct. Future advances will require longitudinal, mechanistic and diverse population-based studies to improve interpretability, reproducibility, and translational utility.