So I checked and the park had no age restriction on it. The main reason people don't do it is because it's $20 per person, not including admission (i.e, it would be about $54 for each adult... not a cheap party). On the other hand, most of my family and several of our friends already had passports, so we wouldn't have to pay admission. And my brother, my sister, and I all have birthdays fairly close together. Thus, we decided to have a family birthday party for all three of us at the park, which we did on December 3, 1995.
After you sign up for the party, they send you a note to let you know when and where to meet. Everybody gets lunch (it's typical so-so Disney fare) and cake and party gifts (a Mickey-shaped pencil, an autograph pad, and some other stuff). They have a bunch of the characters around for autographs and pictures, then they play some games with the kids. We all declined to participate in Simon-Says led by the Mad Hatter, but we did take some pictures of the three of us with some characters and in the general birthday area.
One of the best parts of having your birthday at the park is that each birthday person gets an envelope with one of 4 prizes in it. I got to pilot the Mark Twain, Dave got to ride in the engine of the Train, and Judy got us all reserved seats for the Beauty and the Beast show. The fourth prize is that you get to ride in the driver's compartment of the Monorail (which is the least cool of the four, as you can do this anyway).
Piloting the Twain turned out to be doubly cool, as they gave me a really neat certificate. (Be warned that the image of the certificate is over 200KB. I didn't want to reduce it's beauty by reducing it's size. enjoy!) Since the Twain is on tracks ( or should I say twacks :-), you don't actually get to drive it, but you get to have 3 other people and yourself ride in the wheel house, you get to steer, and even blow the whistle and ring the bell.
Unfortunately, when I got to the Twain, there were already two people reserved before me (other Birthday people) to drive the Twain, so I got on the last boat at dusk, so most of the pictures and movies I took from the wheelhouse didn't come out very well. For the engine of the train, they'd only let 2 people ride, so Dave and his girlfriend went. Since it was dark by that time, the couple of pictures they took didn't really come out. All in all it was a lot of fun, and I'd recommend it for anyone to do at least once... Now if only I can convince my brother and his girlfriend to get married at the park. :-)