Mac OSX’s iCal is able to show birthday entries based on your address book. However, at least in the German localization you cannot read the person’s name at first glance, but only “36. Geburstag vo” – also in the notification center. This post resolves the problem and provides help to show birtdays as “* Frederic Miller (36.)”. Read how.
In Mountain Lion’s finder, go to /System/Library/Frameworks/CalendarStore.framework/…
(or your preferred language pack). Copy the file Localizable.strings to your desktop. Then, open the file in a tool like TextWrangler. You will find the entry:
<key>%@\U2019s %@ Birthday</key>
<string>%2$@. Geburtstag von %1$@</string>
Now adjust the <string> … </string> part to your liking, e.g.
<string>* %1$@ (%2$@.)</string>
which shows each birthday as “* Frederic Miller (36.)”.
Now rename the old file in the system folder to LocalizableBackup.strings and then move the edited file from your desktop into the appropriate system folder. Reboot the system and it should work!