Thank you, sir, that worked. ;)

Just curious, but how did you figure out that the birthday variable wasn't defined in member_start?

1st Step: DEVDEBUG($userinfo[age]), did show nothing
2nd Step: DEVDEBUG($userinfo[birthday])
=> Aaargh, it's formatted
3rd Step: Ok, lets check if there are Hooks in member.php executed earlier.
Bad, there are none
4th Step: OK, so lets use fetch_userinfo which must be called to get the User Info
=> Working

Thank you, sir. Maybe next time I can try this approach BEFORE I start beating my head against the keyboard. ;)

When I grow up, I want to be like Kirby.. ;)