My experiences to date...
'99, my dd turned 6 at WDW. At CP that night they brought her out a card signed by the characters and a special birthday cupcake with a candle. I had noted her birthday when we booked the stay as well as when we checked in at the restaurants.
In '06, she turned 13 in WDW. I mentioned it at the time of booking my room and when I booked each ADR. She wore her birthday button on her actual birthday, as well another button the other days that said..'celebrating my birthday at WDW'..the birthday button said 'Today is my birthday'. She was asked to stand up and be recognized at HDDR on her birthday. I ordered her a special cake at Grand Floridian Cafe for lunch..the server brought it out and had the entire restaurant sing to her. She got a special birthday placemat and cupcake at Chef Mickeys. And a lot of 'happy birthdays' as she walked through the parks.
But, other than those things, nothing to speak of..no free FPs, no free ice cream, no balloons etc.