A prototypical model for an age-structured diffusive population is considered in which individuals are distinguished by age and spatial position. The evolution equation involves a diffusion term for the space variable and a transport term for the age variable supplemented with a nonlocal boundary condition. The linear version of the model gives rise to a strongly continuous semigroup which exhibits the parabolic regularizing effects in the space variable. We determine its asymptotic behavior based on spectral properties of the associated generator. For a nonlinear version of the model we investigate the existence of nontrivial steady states and establish a principle of linearized stability.