Add a new dates format: yyyy/mm/dd
AND/OR:
Make a modification so that a user doesn't have to type the slashes "/" between the numbers when registering their birthdate. So instead of typing yyyy/mm/dd, he can just type yyyymmdd.
What do you think about making selectors there instead of text boxes? So the user will not have to type the birthdate. He/she will simply select values from dropdown lists.