Logic for Inventory Aging Report for Average costing organization

I want to understand how the age will be calculated, as generally the age is calculated from receipt date till as of date (any provided date). But how would you identify which item is utilized/issued?

Lets take an example:

Purchased 1000 units on 1 July 2017

Purchased 500 units on 1 August 2017

Sold 300 units on 15 August 2017

Now on 15 August 2017 the balance is 1200 how would it be identified that 300 units which were sold belongs to (1 July or 1 August ).

We can only calculate if we assume we issue based on FIFO/LIFO.But that's not the case always.

Note: we are not using the Serial or LOT enable items