- Create a civi profile that has
View/Edit Drupal User Accountchecked under "Used For".
- Add the Individual birthdate field.
- Clear drupal cache at admin/config/development/performance (otherwise the tab in the next step won't show up).
- Edit any user record under /admin/people.
- Click on the tab for the profile you made.
Civi and civicrm-drupal-8 version is master.