This study aims to explain why children are impatient. Using a biological framework called the life history theory, the study investigates the evolutionary root of time preference, paying particular attention to childhood. The results show that the biologically endowed rate of time preference is equal to the mortality rate not only in adulthood but also in childhood, reflecting the change in the biological value of survival. Mortality is the baseline for time preference through the entire course of life. These results are consistent with the findings in previous empirical and experimental studies that the discount rate is U-shaped in age, and account for why young children, in particular, are impatient. In addition, the difference in time preference between adults and children provides a biological explanation for the parent-offspring conflict, in which the higher discount rate among children causes parents and their children to disagree over intertemporal allocation of resources in collective decision-making particularly within the household.