WaterGEMS Water Quality Question

If I am imitating an SWTP sending water to a repump station by using a reservoir to send water to a GST, and then using a Booster pump to send it out in the distribution system, how does the water age calculation work here? Does it count the water age to start once it leaves the GST or when it leaves the reservoir?

If it starts counting water age when it leaves the reservoir, is there a way to make it start from the GST?