<p>Sex differences in ageing and lifespan are widespread across taxa, yet their evolutionary causes remain debated. A leading hypothesis proposes that such differences arise adaptively from sex-specific life-history trade-offs, but formal theoretical support is lacking. We therefore develop a mathematical model to examine how these trade-offs shape lifespan evolution across ecological and mating system contexts. Under monogamy, individuals optimise a survival-reproduction trade-off, mediated by mating opportunities in males and offspring production in females. Varying trade-off strengths shows that male-biased longevity emerges across a wider parameter range, consistent with patterns in monogamous species. This asymmetry arises because increased female longevity reduces offspring production, lowering population size and mating rates and thereby constraining selection on female lifespan, whereas longer male lifespan evolves more readily unless mating costs are high. Incorporating density dependence shows that ecological feedbacks disproportionately reduce survival in the intrinsically longer-lived sex, potentially masking evolved differences. Beyond monogamy, we show that mating system variation shapes longevity through competition intensity, determined by the limiting sex, and demographic scaling, where female number sets reproductive output and the selective context for longevity. Overall, we highlight that life-history trade-offs and demographic feedbacks jointly determine the direction and magnitude of sex differences in lifespan.</p>