I treat maximum lifespan as one thermodynamic problem. A body stores biological information, entropy degrades it, and holding it intact costs energy drawn from a finite budget. From that floor I derive one relation, ρ = D·V/M(1−R): aging climbs with damage generation (D) and storage vulnerability (V), and falls with the maintenance budget (M) and the share not spent reacting to environmental threat (1−R). Because no organism can drive all three demands to zero on a finite budget, aging can never reach zero — finite lifespan becomes a requirement, and every species is a differently distorted version of one shape. Tested against comparative data, damage generation holds; membrane vulnerability does not survive control for mass and phylogeny and is left open; the environmental demand is strongly supported once measured as extrinsic mortality rather than stress hormones, on two timescales. I specify the composite test the framework still needs.