Abstract
<jats:p>Background. The dominant clinical manifestation of aging is not mortality but the progressive depletion of functional reserve—the surplus physiological capacity separating independent function from disability. Existing geroscience frameworks describe the molecular processes of aging in extraordinary detail yet offer the point-of-care clinician little basis for prioritizing targets. Objective. To reframe functional aging as a control-systems problem and to define the minimum set of physiological capacities whose preservation maintains functional reserve. Framework. Aging is modeled as progressive instability within a coupled, bidirectional mitochondrial–epigenetic regulatory axis. Candidate capacities were evaluated against three operational criteria—independence, necessity, and modifiability. Results. Five capacities satisfy these criteria: bioenergetic capacity, endocrine signaling integrity, molecular quality control, adaptive (hormetic) stress response, and neuro-autonomic regulation. Each protects the central axis; together they govern the trajectory of functional reserve. Modifiable interventions are mapped onto these capacities in a separate operational layer. Conclusions. The framework yields a causally ordered, clinically actionable, and falsifiable architecture in which functional reserve is the primary therapeutic target and lifespan a downstream consequence. It is testable against existing longitudinal cohorts without new data collection.</jats:p>