Does the creation of a reserve fund for the PAYG pension system increase welfare?

We analyse the effects of the creation of a reserve fund for the PAYG pension system; the fund is put in place before ageing appears, ageing consisting in an exogenous increase in life expectancy. We examine the open economy case, with perfect international capital mobility, and the closed economy case with endogenous capital, wages and interest rate. We show that in the absence of imperfections in all cases welfare is increased, when the generosity of the PAYG system is maintained and when tax pressure is stabilized as well, except in the case of intergenerational altruism.