Every job I have ever worked at has had the same weird tradition. That is, of forcing the employee who has a birthday to make a cake to share with the staff (or buy one if they have more money than time or are male) and put it in the break room, along with bagels and cream cheese. I think this is to spare the unpopular people from the embarrassment of having no one to give them a cake on their birthday. If someone doesn't bring in a cake it shows they are lazy, but at least not un-loved.
If you listen carefully on such occasions (it's always fun to gather with bored but hungry co-workers around the cake) you might hear the birthday person reveal their age. That is the best part. The rest of the day people say "I never believed she was that old" (or young). The world of work is so much fun you can see why people hate to leave it, even if it means spending less time on their yachts.