I've noticed it as well, he probably just changes his birthday in his profile. It didn't say it was his birthday earlier, it just said it was MightyJoe's birthday, now willkillforfood is on there as well.
And they wouldn't really celebrate. By the time you're 100 y/o you'd be thinking "Oh great...another day of being a vegetable. And what's that weird Queeny woman doing sending me a birthday card? She's not the true monarch. Last time I checked it was Edward!!".