The Rate of Living and the Threshold Theories of Agingare two contradicting approaches used to explain experimentalfacts about aging in fruit flies. In this paper we suggest anapproach that unifies these theories and removes thecontradiction. The approach involves quantitative description ofthe oxidative stress theory of aging, which is presented in theform of a mathematical homeostatic model. The crucial variable inthe model is called 'homeostatic capacity', which is analogous tothe classical notion of vitality. We model the process of agingas the age-related accumulation of damage produced by oxidativestress, which reduces the homeostatic capacity of the organism.The model is tested with the experimental data obtained in theclassical experiments by Maynard Smith in 1958–1963. Ourhomeostatic model explains the well-known results of theseexperiments more accurate than any one of the early theories ofaging. We form an hypothesis about the mechanisms underlying theresults observed in the experiments and analyze a possibleinterplay of these mechanisms. Our virtual replication of MaynardSmith's classical experiments demonstrates that mathematicalmodeling can be a powerful tool to reveal and investigate theinherent genetic and physiological processes underlying the dataobserved in complicated insect experiments.