The developmental trajectory of delay discounting has been debated for over 30 years, with empirical findings often inconsistent. The controversy stems partly from the absence of formal models to guide empirical investigation and theoretical construction. We proposed a normative model of delay discounting across the lifespan, building on the work of Sozou and Seymour (2003), which emphasizes the tradeoff between mortality, fertility, and parenting in determining delay discounting. We simulated the model across varying parameter spaces and identified a U-shaped association between age and delay discounting, with stronger parental investment postponing the turning point of the function and reducing the overall discount rate. Empirical data supported these predictions, demonstrating a U-shaped age effect and highlighting the role of parental care motivation in explaining cultural and age-related differences in delay discounting. The results suggest that variance in delay discounting can be understood as a rational adaptation to life history tradeoffs.