The time that rain and melting snow take to flow into a river or to a location under the ground impacts nutrient and contaminant transport, and is therefore important to sustaining healthy humans and ecosystems. These ‘water ages’ of streamflow and groundwaters have remained poorly understood. Here, we use stable and radioactive isotope compositions of rain, snow, groundwater and streamflow to map the relative amounts of younger versus older water (i) flowing in rivers, or (ii) discharging from groundwater wells. First, our research shows that ~1/3 of global streamflow is recent rain or snow that fell and flowed to the stream in less than ~2.3 months. This young streamflow is detectable in the great majority (~90%) of surveyed streams, implying that most catchments can convey precipitation to the stream channel quite quickly, possibly leaving little time to detect some soluble pollutants before they reach aquatic ecosystems. Second, we show that most of the fresh water on the planet is ancient water that has been stored underground form more than 10,000 years. While vast, these ‘fossil’ groundwaters are also shown to be vulnerable to modern-era pollutants, emphasizing that both water quality and sustainability should be considered when managing these deep water resources.