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The purpose of this paper is to analyse the effect of the irreversibility of the retirement decision on labour supply after sixty. According to theory, when the retirement decision is an absorbing state, people tend to remain longer on the labour market to be able to benefit from changes in preferences or economic environment favourable to participation. Simulations performed on a sample of individuals with heterogeneous careers illustrate this result. When pensions schemes are actuarially fair, the irreversibility of the retirement decision leads to higher participation rates in the initial years of the potential retirement period. Moreover labour supply around sixty increases with uncertainty. Simulations show that the impact of an increase in uncertainty on participation rates depends to a large extent on pension scales. Whereas the effect is considerable with actuarially fair systems, it is almost insignificant in the French institutional setting where individual choices are highly constrained by the rules.