Description: Accurate determination of the appropriate time to retire and replace a physical asset is important for large ensembles of such assets when taken as a whole, and concerns the fate of tens of trillions of dollars in assets, such as airplane engines, vehicles, or any other kind of physical asset susceptible to obsolescence. A decision model to accomplish this is derived using the concept of actuarial present value applied to an economic theory of cost minimization. Since this is a theory coupling investor wealth creation with the long-term optimization of the use of scarce resources, it can also be offered as an economic model relevant to sustainability.