Two of the most important qualities of a constitution are legitimacy and longevity. Fittingly, we have theories describing the grounds of constitutional legitimacy, and empirical work on the predictors of constitutional longevity. However, the linkage between these two qualities has been less well addressed. In this article, I begin from the premise that contemporary constitutions derive their initial legitimacy from the democratic credentials of the drafting process. However, the ongoing sources of constitutional legitimacy are less well understood. Does the legitimacy of the founding moment sustain the constitution, or is it replaced by other sources of legitimacy? Building on a case study of South African, I develop an argument about the shifting grounds of constitutional legitimacy, and the role of generational changes in creating periods of constitutional instability.