Groundwater age, defined as the time since the groundwater was recharged, is useful for estimating the groundwater renewal. The radiocarbon (14C) activity has been widely applied for groundwater age dating since 1950s. However, the application can be complicated by other 14C-free carbon sources, which will dilute the initial 14C content at groundwater recharge. Several modifications/corrections models have been introduced to obtain the dilution factor, on the basis of carbon isotope and water chemistry mass balance. However, these models do not entirely account for the influence of deeply derived (endogenic) carbon source and are hence not applicable for dating groundwater over endogenic CO2 degassing areas. We developed a modified 14C groundwater dating model to include three carbon end members of biogenically derived sedimentary carbon, carbonate carbon and deep crust and/or mantle carbon, combining the principles of water chemistry with carbon isotope mass balance to quantify the dilution factor. The new model, applied to aquifers in California, predicted younger groundwater age and less mass fraction of fossil groundwater by 15~25% and 5%~10%, respectively, compared with the predictions of the traditional method without endogenic CO2 considered. In principle, the proposed method here can be applied for groundwater age estimating over other regions globally, where the aquifers are affected by endogenic CO2. However, its application requires local geological, hydrogeological and geochemical situation to be identified in advance.