One introduces the Green's function allowing evaluating steady-state water age and partial water ages aimed at assessing ventilation rates in the World Ocean. Elementary physical intuition and Green's function based developments suggest that the ratio of any partial age to the age should scale as the ratio of the volume of the related subdomain to that of the whole domain of interest. Analytical solutions derived in the framework of a water column model fail to fully support this idea. However, potentially interesting properties are established that pertain to the behaviour of the abovementioned time-independent ages in the vicinity of the water-air interface.