Hi,
The implementation is very simple. Every cell in the domain is given a value of 0 at the start of the simulation, and the end of every time step the age in every cell is then increased by the timestep del_t. New water coming into the domain through inflows has a value of 0. The water is then advected around the domain and mixed vertically the same as any scalar. So it is almost the same as a passive scalar but it increases in time.
CHris