The p(p,e^+ \nu_e)^2H reaction rate is an essential ingredient for
theoretical computations of stellar models. In the past several values of the
corresponding S-factor have been made available by different authors. Prompted
by a recent evaluation of S(E), we analysed the effect of the adoption of
different proton-proton reaction rates on stellar models, focusing, in
particular, on the age of mid and old stellar clusters (1-12 Gyr) and on
standard solar model predictions. By comparing different widely adopted p(p,e^+
\nu_e)^2H reaction rates, we found a maximum difference in the temperature
regimes typical of main sequence hydrogen-burning stars (5x10^6 - 3x10^7 K) of
about 3%. Such a variation translates into a change of cluster age
determination lower than 1%. A slightly larger effect is observed in the
predicted solar neutrino fluxes with a maximum difference, in the worst case,
of about 8%. Finally we also notice that the uncertainty evaluation of the
present proton-proton rate is at the level of few \permil, thus the p(p,e^+
\nu_e)^2H reaction rate does not constitute anymore a significant uncertainty
source in stellar models.Comment: accepte