To the OP:
Don't flip out, that'll only scare your daughter when she sees how uncomfortable this makes you. Its a natural thing, embrace it.
9 is not as early as you think. My mom got her first period when she was either 9 or 10. I got mine between grades 6 and 7, I think I was 11.
You won't freak her out if you explain the facts of life to her. Don't keep it a mystery, I feel that that only confuses young girls. My mom wouldn't talk to me about it, in fact, I remember when I first asked her what a pad does after watching a commercial about them, and when I suggested blood, she denied it! And dropped the conversation! Within the next year, I got my period, and I was so mad at her for lying. Stupid thing to do... -_-
Don't wait for your daughter to hear all sorts of strange things from the kids at school, take a deep breath, remember she will quickly grow into a beautiful young woman in the next 8 years, and be as helpful and understanding as possible.
Don't think of it as letting her go, you're opening her mind up to the real world, to what life is about. You still have a number of years with your baby girl before she graduates from high school
I think that you should tell your daughter soon where babies come from as well. I think it's important girls and boys hear it from their parents before they hear it said poorly in school. In the catholic school board where I live, kids start sexual eduation in grade 5...I was nine in grade 5.
As for the "belief" that girls are maturing earlier, I think its a load of steaming shit. I have seen no real evidence that the median has dropped in the last 50 or 100 years. I think its just another scare tactic issued out like so many others. *shrugs* I've read some really excellent sex ed books by renowned doctors, and nothing implies that the food we eat, or the drugs we take, have sped up puberty. The body's internal clock, and the pituitary gland, do all the work for you. As far as I know, and I've been taking sex ed and human anatomy courses for years now, nothing but human genetic defects can fuck up one's sex cycle.
I agree with what some of the woman have said in this thread about sharing your experiences from growing up then with her. Knowing that her mommy and other girls have the same feelings, the same concerns and questions, will make her feel more comfortable in her body as it changes. Because my mom wouldn't talk to me, I felt like one ugly duckling all through elementary school...and I wouldn't wish how I felt on any child.
The one thing I do think that has changed in the last 100 years is the use of birth control pills...all of that peed out synthetic estrogen is not doing the male sex any good. Sterility is on the rise, and that is what concerns me... :S