1. Summary The Dis-Economies of Scale Theory of Aging (DESTA) proposes that senescence in animals is an adaptive, programmed consequence of growth-termination driven by unavoidable diseconomies of scale. Growth-termination and senescence evolved and persist as the final major acts of the developmental process in terrestrial animals. Because individuals must reach a species-appropriate adult size and developmental state before reproducing, sexual selection reinforces the timing and form of growth termination and the physiological configuration it creates. DESTA argues that senescence benefits the growth-terminated individual because it enhances the probability that populations of its progeny will persist over deep transgenerational time. Driven by sexual selection, the regulatory system that halts growth continues to operate after maturation and keeps adjusting the body’s metabolic, endocrine, autonomic, and circadian settings throughout adulthood. Its continued activity gradually shifts these settings away from the high-vigor state of early adulthood and produces the changes recognized as senescence. Because sexual selection reinforces the developmental and physiological states established at maturity, the same system that stops growth also determines how adult vigor declines with age. Senescence therefore arises as a direct continuation of this lifelong regulatory operation, not from developmental drift, relaxed selection, or accumulated damage.