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Aging drives molecular changes that impair cellular and tissue function. Lipids are central to membrane structure, signaling and energy storage, yet their remodeling during aging remains unclear. Here, using cross-species, multi-tissue lipidomics, we identify elongation of lipid acyl chains as a conserved hallmark of aging across mice, Caenorhabditis elegans, Drosophila and humans. Similar lengthening occurs during the progression of human heart disease, whereas dietary restriction shortens cardiac lipids in mice. Integrated analyses reveal that aging is characterized by a shift toward longer lipids accompanied by depletion of shorter species, indicating ratiometric remodeling. Following this, we identify the lipid remodeler Plb1 as a regulator of this process, with expression correlating with lifespan in mice and genetic analyses supporting a causal role in human frailty. In C. elegans, Plb1 knockdown reverses lipid elongation and extends lifespan in a lipid-length-dependent manner. Together, these findings establish lipid chain length remodeling as a conserved, actionable hallmark of aging.