If you must do it like that, try this regex:
^((?<day>\d{1,2})/(?<month>\d{1,2})/(?<year>(?:\d{4}|\d{2}))|(?<age>\d{1,3}))$
But me? I'd use two controls, and a radio button. And one of the controls would be a date picker.
[Edit]System added spurious closing tags - OriginalGriff[/Edit]