This paper discusses low mass, metal poor stars, presenting new evolutionary computations which take into account the inward diffusion of both He and heavy elements during the H burning phase. The investigation has been extended to He burning models, discussing the luminosity of the Zero Age Horizontal Branch (ZAHB) models originated from progenitors with efficient element diffusion. We find that diffusion of heavy elements plays a non-negligible role in the evolutionary paths of the model. One finds that element diffusion has a sensitive influence on evolutionary models, a less significant effect on the isochrone turn off and, finally, a rather marginal impact on the difference in luminosity between the ZAHB and the isochrone Turn-Off (TO). The consequences of these results on current evaluations of globular cluster ages are discussed in connection with possible sources of errors.