The young water fraction (F_yw), the proportion of water younger than 2-3 months, was investigated in soil-, ground- and stream waters in the 0.56 Km² sub-humid Mediterranean Can Vila catchment. Rain water was sampled at 5-mm rainfall intervals. Mobile soil water and groundwater were sampled fortnightly, using suction lysimeters and two shallow wells, respectively. Stream water was dynamically sampled at variable time intervals (30 minutes to 1 week), depending on flow. A total of 1,529 ¹⁸O determinations obtained during 58 months were used. The usual hypothesis of rapid evapotranspiration of summer rainfall could not be maintained, leading to discard the use of an “effective precipitation” model. Soil mobile waters had F_yw up to 34%, while in ground and stream were strongly related to water table and discharge variations, respectively. In stream waters, due to the highly skewed flow duration curve, the flow-averaged young water fraction (F*_yw) was 22.6%, whereas the time-averaged F_yw was 6.2%. Nevertheless, both F*_yw and its exponential discharge sensitivity (S_d) showed relevant changes when different 12-month sampling periods were investigated. The availability of S_d and a detailed flow record allowed us to simulate the young water fraction that would be obtained with a virtual thorough sampling (F**_yw). This showed that underestimation of F*_yw is associated with missing the sampling of highest discharges and revealed underestimations of F*_yw by 25% for the dynamic sampling and 66% for the weekly sampling. These results confirm that the young water fraction and its discharge sensitivity are metrics that depend more on precipitation forcing than on physiographic characteristics, so the comparisons between catchments should be based on mean annual values and inter-annual variability. They also support the dependence of the young water fraction on the sampling rate and show the advantages of flow-weighted F*_yw. Water age investigations should be accompanied by the analysis of flow duration curves. In addition, the simulation of F**_yw is proposed as a method for checking the adequacy of the sampling rate used.