I had this question a few months back! My son was in 3rd grade and 9 years old. With advice from his dr. and others, we decided to have the "talk" with him. His dr. suggested a book called, "What's the Big Secret." The doc said my son was a little old for the book in that it is drawn with cartoons, but it was perfect! I checked it out from the library, and my husband and I sat down one night with him and went through the book.
At first, my son was giggling at the pictures, but then he was very interested in what we had to say. He had lots of questions. I really thought it was going to be much more uncomfortable than it was. I'm so glad we did this! I'd much rather him hear the truth from us than to hear wrong information at school. We told him that some kids may make up rude words regarding this, but that he will know the truth.
Regarding puberty, I plan to go the bookstore and do some research!
Not sure how old your child is, but check the book out yourself first and see what you think!