Population aging has intensified the search for biological measures that can explain why individuals of the same chronological age differ so markedly in health, function, resilience, and disease burden. Geroscience has provided a powerful framework by linking shared mechanisms of aging to multiple age-related conditions, but translation to clinical practice remains incomplete. Aging clocks and circulating biomarkers have improved biological-age estimation, yet prediction alone does not establish biological meaning or clinical utility. Emerging evidence instead supports a view of aging as a heterogeneous and multidimensional process shaped by organ-specific vulnerability, cumulative stress, multimorbidity, and declining resilience. The field should therefore move beyond single biomarkers and universal aging clocks toward gerodiagnostic approaches that characterize individual aging trajectories and their modifiability. Progress in gerotherapeutics will similarly require closer integration of mechanism, measurement, and intervention. The ultimate goal is not merely to quantify aging, but to identify actionable biological processes whose modification may preserve function, delay multimorbidity, and extend healthspan.