Considerations of chemical evolution have been used to generalize Schramm and Wasserburg's formalism for deriving a mean age of the elements in the Galaxy at the time (T) when the solar system formed. Comparison of the equations of nucleochronology with those of chemical evolution reveals that the earlier result is restricted to models in which the stellar birthrate varies linearly with the mass of gas in the system, and moreover to those in which the ratio of birthrate to gas mass defines a time-constant much greater than T. Relaxing the latter assumption, in particular (which seems necessary in the light of independent evidence), considerably increases the model-dependence of any determination of T from nucleochronometers. It is found that Schramm and Wasserburg's quantity - A (which can in principle be evaluated from abundances and nuclear data on the radioactive elements) does not necessarily lie between T/2 and T, as it does in the restricted set of models; instead, it may lie anywhere between 0 and T, depending critically on the nature of the evolutionary model. In particular, if inflows of metal-poor gas have significantly affected chemical evolution in the solar neighborhood, T could be considerably greater than - A. This quantity is shown to be equal to the mean age of stable elements in the gas at time T. It is concluded that the nucleochronometers provide a model-independent lower limit to the time T, but that derivation of a more precise age of the Galaxy from radioactive time scales will require detailed understanding of its chemical evolution. Subject headings: abundances - Galaxy, the - nucleosynthesis