1. Aging as a Time-Dependent Deleterious Consequence of Being Alive

What is aging? Biological aging is the time-dependent deleterious consequence of an imperfect system being alive, wherein the accumulated damage from internal processes and exposures leads to functional decline and ultimately mortality. Its essence is the rise of the deleteriome, the cumulative burden of deleterious changes in a live organism. It is also useful to consider outcomes of biological aging, e.g., functional aging, morbidity, demographic aging, as well as physical aging (aging of inanimate systems). Biological aging is not reducible to a single molecular mechanism, a list of hallmarks, or any one pathology. It is the net result of imperfect molecular, cellular, and organismal processes acting over time (2–5). This definition ties aging to a measurable trajectory (deleteriome/biological age progression), outcomes (loss of function, disease vulnerability, mortality risk), and interventions (factors that slow, accelerate, reverse, or reset that trajectory).