In the save files, each character has a date = "...." entry which shows his date of birth (e.g. date = "10600508", in the form "yyyymmdd"). The game calculates age from that. Note that the numbers for months and days actually start from zero so January 1st, 1060 would be 10600000.
In the scenario files, characters have a similar birthdate entry, except there the years/months/days are specified separately (months by their english names rather than numbers) and days start from 1 rather than 0.
It's odd, I know.