This article focuses on the study of an age structured SEIRS epidemic model with a vaccination program when the total population size is not kept at constant. We first give the explicit expression of the reproduction number [MATHEMATICAL FORMULA] in the presence of vaccine ([MATHEMATICAL FORMULA] is the exponent of growth of total population), and show that the infection-free steady state is linearly stable if [MATHEMATICAL FORMULA] and unstable if [MATHEMATICAL FORMULA], then we apply the theoretical results to vaccination policies to determine the optimal age or ages at which an individual should be vaccinated. It is shown that the optimal strategy can be either one- or two-age strategies.