I won't tell you whether or not to tell your daughter. However, last spring, my 11 year old told me he didn't believe in the leprechauns that come to our house every St Patty's Day. I didn't reaffirm his suspicions right away, during the conversation, but then told him that, yes, I was the leprechauns. I ended up bawling all night long!! I knew that meant no more Easter Bunny, Santa, etc. I couldn't believe I just burst the bubble!! My heart broke. I asked him the next morning if he was sad,now that he knew. He said.."Not really". But, I know a part of him was. Anyway, the reason I DID tell him, was because I was the oldest of 4 (just like him) and I remember when my Mom let me help her with the Santa gifts on Christmas Eve. It's a memory I will cherish forever. My sibs were younger (4- 12 years difference from me), but I thought being able to share that with my oldest would be fun. He has already helped me with some shopping, and it's going well. We'll see about Christmas Eve.....
My 10 year old still believes and doesn't show any signs, at all, that he is starting to doubt. For him, I will wait until he is really questioning it all, before I tell him. If he gets teased by friends, that may speed up the process, but I won't just come out and take away the magic. I have heard that some kids get mad after they learn the truth, but you know your daughter and whether or not that's the case. I eventually learned the truth, and don't remember it being traumatic or getting teased. It's your call - but once you put it out there, you can't take it back.
Good luck!