Aging is one of the most fundamental and least understood biological phenomena. Prevailing theories attribute senescence to genetic programming, telomere attrition, oxidative damage, and evolutionary indifference to post-reproductive survival. Here we propose a complementary thermodynamic framework: aging arises as the inevitable consequence of the cessation of organismal growth. During growth, increasing cellular mass generates accelerating energy dissipation, rendering the organism thermodynamically favorable as a dissipative structure in the sense of Prigogine. Upon growth arrest, dissipation stabilizes and no longer increases, reducing the thermodynamic advantage of maintaining the structure. Energy flows through the system diminish in efficiency, initiating progressive structural degradation — aging. This hypothesis is supported by cross-species metabolic data and explains a key anomaly: thermoconforming organisms (ectotherms and the naked mole-rat), which do not expend energy on thermoregulation, dissipate far less absolute energy and live disproportionately longer than their body mass would predict. We propose that aging is not a program but a thermodynamic inevitability imposed by the finite nature of growth in a resource-limited environment. Keywords: aging, thermodynamics, entropy, dissipative structures, Prigogine, energy dissipation, metabolic rate, thermoconformers