Point defect migration is considered as a mechanism for aging in ferroelectrics. Numerical results are given for the coupled problems of point defect migration and electrostatic energy relaxation in a two-dimensional domain configuration. The peak values of the clamping pressure at domain walls are in the range of 106 Pa, which corresponds to macroscopically observed coercive stresses in perovskite ferroelectrics. The effect is compared to mechanisms involving orientational reordering of defect dipoles in the bulk of domains. Domain clamping is significantly stronger in the drift mechanism than in the orientational picture for the same material parameters.