The reorientation of defect dipoles and the drift of free charge carriers are the most prominent microscopic mechanisms under discussion to provoke the aging effect in ferroelectrics. These two mechanisms are contrasted taking into account the influence of grain boundaries in a polycrystalline material. For the drift model, clamping pressures on domain walls only depend on geometry and on the transport properties of the mobile defect charge carrier independent of its electronic or ionic nature. For a numerical example clamping pressures as a result of drift of oxygen vacancies are determined in BaTiO3. They range from 106 to 107 Pa corresponding to experimental values.