The Aging Analysis shows the length of time since inventory has been received. The report assumes first in-first out inventory management.
In order to compute the age of a product we walk through the historical stock backwards subtracting intakes from the current stock until we reach zero. Each intake increases the value of new stock in the corresponding time window.
The aging report is computed based on the historical stock and it won't work immediately after you connected your store to Inventory Planner.