Whenever you Open a New Database, MM will ask you to specify the Age-Up-Date to use to calculate each Athlete's age. That way when you enter an Athlete's birth date, MM will automatically calculate his/her age. If you are not entering birth dates, then it makes no difference what date you enter for the Age-Up-Date.
If later you find that you want to change the Age-Up-Date and have MM recalculate each Athlete's Age based on this new Age-Up-Date, from the Main Menu Bar click on Set-up and then Athlete/Relay Preferences. Now you may change the Age-Up-Date (Date for computing ages) on the screen. MM will ask if you want the ages of the Athletes recomputed. Click the Yes button and MM will compare the new Age-Up-Date with the birth date of all of the Athletes and store the new updated age in the Database.