Ds is 14 years old. I had a hysterectomy (I had a huge fibroid) when he was 4 1/2, and we discussed menstruation prior to that in a very easy, matter-of-fact way. He was never bothered about the blood or anything.
I've explained to him everything to do with puberty of both sexes (as well as menopause!). He understands menstruation and why it happens. No big deal, though he is glad he is a guy!. We've talked about boy-to-man developments quite openly and he understands all the aspects of reproduction and sex (as well as the consequences!).
Dh is a wildlife biologist, so this subject is no different than any other, in our home, and has been (and, is) discussed openly. If he has a question, he asks and we give him honest answers.
Ds is big on privacy now and we respect that.