J.D.
I started piano lessons at the age of 6. I started asking to take them at the age of 4 because my mom, brother and sister all took them but the teacher said that she would only start teaching me after I had learned how to read. I'm not sure if you know how to play, but even though piano music and the written english language are 2 different languages, the comprehension skills that are required to understand the relationships between the written representation and the actual sound are the same. There are plenty of people that start their kids off at a much younger age, like around 3, but the methods for really young kids are not based on reading music, but rather on ear training (they listen to the music to be performed and mimic it on the instrument.) This makes sense if you consider that you learn how to speak before you learn how to read. There are pro's and con's to doing it either way, but the best way to find out for sure is to expose your daughter to piano music as much as possible, go see concerts with people playing piano, etc. If she develops an appreciation for it, she may have an innate interest (like I did) that will make it easier for you to choose the path that suits her best. I was the first child in my kindergarten to learn how to read because I was motivated to take piano lessons! Now I have a degree in music and have written hundreds of songs . . . all because I was exposed to it at a very young age.
Good luck!
J.