It is very easy if you entered inventory with serial number

Just go to report to company and financials to Balance sheet then open inventory stock

you will see purchase stock as well sold stock  customize serial numbers

export it on excel 

select serial number column and highlight duplicate then filter with color no fill

you will find the unsold closing stock serial numbers. now write todays date at any place and apply formula today date with dollar sign minus bill date and drag down then select numbers you will see the correct aging

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