The discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' observed density, and variable water flow rates within the rocky core to constrain duration of active serpentinization, and therefore, the maximum age of the liquid water circulation in the core. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H 2 production rate in all model cases (> 1 ×10 12 moles yr-1) is several orders of magnitude greater than the minimum H 2 release rate calculated from the observed H 2 in Enceladus' plume (1 ×10 9 moles yr-1), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H 2 production for up to ~75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H 2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus developed only very recently-even as recent as within the past 100 Myr.