We're going to be at Mickey's World during the week of my daughter's 8th birthday. My understanding is that CM's *may* do special things for birthday celebrants, so you have to make them aware that it *is* your birthday. I think they may bring the birthday celebrant a cupcake with a candle, when eating at a Disney TS restaurant, and you might find some extra stuff in your Disney resort room when you return. Key, I gather, is to let everyone from the resort desk to the guy/girl at the restaurant podium know that it is your special day. I have noted that with the reservation folks when I made lodging and meal reservations, and will "remind" them when we get there. I think that the odds are good that if you have an "It's My Birthday" button (available at Guest Services) on, you might get some preferential treatment at times, and for sure some extra recognition.
Anyway, that's what we hear.
Good luck, and happy 21st. (I can *almost* remember being 21..sigh...)
~YEKCIM