P.H.
My 11 1/2 yd old daughter is maturing at what I want to think is an astonishing rate, but when compared to her peers (5th grade graduation last week!) they all seem to be in about the right place. Our pediatrician suggested that about 2 yrs 'ish after the first notice of breast buds, the menstral cycle will start- however that's just a grand, general approximation. I've also had a pediatric nurse friend of ours mention to me that the preemie babies seem to start maturing sooner than their full term peers for some unsubstantiated reason (simply her experience in general) and since my daughter was born at 26 weeks, I've been watching for this as well.
All in all, we started openly answering questions whenever they were asked, in a casual way as to make it not some big, serious event. My mom never ever spoke to me about it, and I remember being terrified. I certainly don't wish that experience on my daughter. She has seen my supplies under the bathroom sink and asked what they were for. I did explain that every body has a general cycle of growing and maturing, and part of that general cycle includes the body preparing to have a baby. If a baby isn't there, then the body simply cleanses it away until another month passes. It's all a part of growing up and becoming an adult.
Now that she's 11 and seen "the movie" this past year, she seems rather comfortable and mentally prepared for it to happen. We also have the American Girl book, "The Care and Keeping of You". If you aren't familiar with it, it covers many topics from deodorant to friendships, not just menstrual cycles.
I've also been very careful to let her know that she can also talk to dad about anything she can talk to me about. I encourage her to have a strong relationship with her father as I so value the relationship I have with mine. She was a bit hesitant at first, of course, but I reassured her that both boys and girls need to learn about their bodies and how they work, and as they continue through school and health class, they will learn all that her parents know. There should be no shame related to her maturing and so I want to keep those doors of communication open as wide as possible!
Good luck - not sure if I answered exactly what you were asking, but hope it helps!