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• We summarize field observations of 14C age showing obvious GW aging in two basins. • Pumping causes not only rejuvenation, but also aging of GW in layered basins. • Although GW rejuvenation is larger in area, aging is greater in magnitude. • We identify two new mechanisms of pumping-induced GW aging in layered basins. • Topography-driven flow is the main cause of GW aging in the discharge area. As an indicator of the renewability of a groundwater basin, the spatial distribution of groundwater age has attracted intensive research. As reported in the literature, 3H measurements in 74 US aquifers showed pumping-induced groundwater rejuvenation, whereas 14C ages in two heavily developed aquifers in France and China indicated pumping-induced groundwater aging. Unfortunately, the mechanisms leading to the seemingly opposite behavior of pumping-induced aging and rejuvenation remain unclear. In this study, we conducted numerical simulations of groundwater age in layered aquifer-aquitard systems under different pumping scenarios and aquitard permeability to show that pumping-induced groundwater rejuvenation and aging always occur simultaneously. Groundwater rejuvenation widely occurs throughout the basin wherever the well is located, whereas groundwater aging mainly occurs to the downgradient parts of the basin. Although groundwater rejuvenation is larger in area, groundwater aging is greater in magnitude, and the magnitude of aging increases with pumping intensity and duration. Moreover, when pumping occurs in the discharge area, aging could occur in almost the entire aquifer to the downgradient of the pumping well, and the magnitude of aging is larger. The large area and large magnitude of aging in the discharge area is a direct result of upward leakage of old groundwater from underlying aquitards/aquifers, which is controlled by the topography-driven groundwater flow. By demonstrating the importance of topography-driven groundwater flow and underlying formations in groundwater age redistribution, this study has implications for determining the bottom of a groundwater flow and solute transport model and interpretation of various tracers. [ABSTRACT FROM AUTHOR]