The age is calculated as date difference at local time. By default the "last modified" file stamp is taken to calculate the age (see options /created, /modified and the note about file stamps).

Note about file stamps:
Usually you would not expect a file to have been "modified" before it was "created".

However, when a file is copied, for example, the new copy is "created" at the current date (obviously), whereas the "last modified" stamp is taken from the original file, because it is supposed to refer to the contents - not the physical file.