LIe congrès annuel de l'Association française de science économique 2002 : Développements récents de l'analyse économique.Accessible sur Cairn : http://www.cairn.info/revue-economique-2003-3-p-561.htmNational audienceWe modify the two period OLG model in order to distinguish the effects of individual ageing from changes in birth rate. We characterize optimal pension and retirement age policies in a dynamic framework. We consider a mixed pension scheme, fully funded but offering agents an actuarially fair choice of their retirement age, and show that it is able to implement the optimal solution. Numerical simulations describe the dilemmas which France currently faces.Nous enrichissons le modèle à générations habituel de manière à distinguer ...