I celebrated my 20th birthday on Dec 26 of this past year at Disney.
To make it really special, buy a "Birthday Cake Hat" at any of the theme parks. They are only offered at welcoming kiosks outside of the park, even before you put your ticket in to enter. They are multi tiered cakes and the candles on top light up when you push a button. While wearing the hat, every single cast member and many, many guests, wish you happy birthday's all day long and embarrass you a little. Also, some CMs gave me some free souvenirs and ice creams...nothing big tho.
Also, every park has a "birthday" pin for free. I just asked for mine at the turnstiles. Also, at the turnstyles, you can talk on the phone to Goofy at Epcot, Mickey ands Minnie and MK, and other charace=ters at the othe rparks. They sing to you on the phone and have a mini convo. It's a tape recording, but is still nice and funny.
Have fun!