I got this book from the library for my son and it was great. It covered everything and was very straightforward and clear. We also watched the video together. Some of the ways things were described irked me and I didn't feel like it was absolutely necessary to discuss how an orgasm feels, but all in all it was a good video. My son seemed to like it and understand it.
My daughter 14 and son is 12, so Im long past having this conversation with them!!! Either way you do it, its going to be akward! I dont think there is an exact formula on how old, and how much detail. You just have to play by ear on what you think is right!