Well there is no Generic solution for your problem/query as WordPress is not Specifically made Biography directory.
Solution will depend on the approach you have followed.
Have you created Custom User Role and add biography user under that user role OR Have you created Custom Post Type for Biography and add posts under that ?
If its User then you must be storing Date of Birth of User in user_meta table OR if its Custom Post Type then you must be storing Date of Birth in post_meta table.