Let ( [ B = YourTable::BirthDateField ;
T = Get ( CurrentDate )
Year ( T ) - Year ( B ) - ( T < Date ( Month ( B ) ; Day ( B ) ; Year ( T ) ) )
Substitute your birthdate field in place of "YourTable::BirthDateFiel" and make sure that this is a field of type date. Also make sure to select unstored as the storage option for this calculation field or the age will not automatically update as time passes.