There is a discrepancy in the water age results for models with short pipes and short travel times. For instance, water age may be rounded up. For instance in a pipe next to a reservoir with a flow travel time of 3 minutes, the water age may display an age of 6 minutes.
In cases where the travel time is shorter than one tenth of a hydraulic time step (typically about 6 minutes, if the hydraulic timestep is set to one hour), you may need to adjust the water quality time step option, under Analysis > Calculation Options. Generally speaking the most accurate results will be obtained from using a water quality time step that's less than or equal to the shortest pipe flow time.
Understanding the Water Quality Time Step