  The principle of linearized stability and instability is established for a
classical model describing the spatial movement of an age-structured population
with nonlinear vital rates. It is shown that the real parts of the eigenvalues
of the corresponding linearization at an equilibrium determine the latter's
stability or instability. The key ingredient of the proof is the eventual
compactness of the semigroup associated with the linearized problem, which is
derived by a perturbation argument. The results are illustrated with examples.
