I have a 10 year-old daughter. I started talking with my daughter about her body and such as soon as she was able to ask questions, which was about the time she learned to talk... I do not agree with the concept of "the talk". We need to have an open, on-going, conversations with our children about our bodies, their functions, puberty, relationships, respect, and sex. I don't think a week goes by where she doesn't ask me a new question now that puberty is in progress for her. Some of her classmates already have their periods, and we're expecting hers within this next year, based on the signs.
The American Girl "The Care and Keeping of You" books are excellent. They have helped her ask good questions and be aware of the changes she should expect. It's nice to have her own resource and not just me to find out these things. They do no cover sex, however. The "It's so Amazing" and "It's Perfectly Normal" books are pretty good for that topic.
I do not tell her that she can't talk with her friends about this topic. That would make it seem somehow secret or dirty. She needs to know that all this is normal, natural, and that all her friends are going through the same changes.