I am assuming these people are alive and reside in Australia. If they married 50 or more years ago in NSW, their dates of birth may well be noted on the marriage registration, so not just age, but actual date is recorded on NSW from mid 1960s.
If male their dob will be on the compulsory national service papers held under limited access rules at Australian National Archives. - two conscription Acts involved, one pre-dates the 2 year service involving Vietnam tours.
If your people of interest are deceased, but you know their year of birth and only want a date for entering their details on your own family tree, then perhaps use 1 January of each year, unless you can discreetly ask their siblings or their children for the day and month.
JM
