What year were you in school? Why haven't you been celebrating birthdays after those periods? Personally, I would like to celebrate my birthdays almost every year because it is never easy being alive and kicking.
It was in my final year that I had that birthday celebration last. I just lost a sense of touch or connection with the whole birthday thing, so I just slowed down completely with it. Don't get me wrong, I'm happy for being alive but throwing a party for it now doesn't make any sense again to me.