In a class of experiments using extended planar sources, the age of fission neutrons is calculated by “correcting” the measured second moment of the flux through the use of a series in the higher flux moments. In this paper, the “correction” is generalized to include terms in addition to the leading term of an eigenfunction expansion of the neutron source distribution. In the generalized correction series, expansion coefficients are shown to be series themselves, which cannot be shown to converge in general. Examination of physically reasonable examples, one of which included only the effect of the energy-dependent extrapolation length of a published experiment, reveals divergences in the series for all expansion coefficients but that of the leading term in the correction series. Since the assumption of an energy-independent extrapolation length was central to the derivation of the correction series in question, this indictment is quite general.