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This paper lays the groundwork for a theory of time allocation across the life course, based on the idea that strength and skill vary as a function of age, and that return rates for different activities vary as a function of the combination of strength and skills involved in performing those tasks. We apply the model to traditional human subsistence patterns. The model predicts that young children engage most heavily in low strength/low skill activities, middle-aged adults in high strength/high skill activities, and older adults in low strength/high skill activities. Tests among Machiguenga and Piro forager-horticulturalists of southeastern Peru show that males and females focus on low strength/low skill tasks early in life (domestic tasks and several forms of and fishing), switch to higher strength/higher skill activities in their 20's and 30's (hunting, fishing, and gardening for males, fishing and gardening for females), and shift focus to high skills activities late in life (manufacture/repair, food processing).