Probably 65, as 45 was my vampire age, and I have enjoyed the time since that age. I am not sure I'd like to go back to fertility though. Think about it, say you take it at 40, menarche around 15 and menopause around 55, that's periods for 60 years instead of 40 years, and twenty extra years you might get pregnant.

I've had some time to think on this, I still say probably 65. I waffled for awhile because I would like to have the build I had before the last 2 children, but keep the kids, the ten pound wonder blew out my abs and skin which happily bounced completely back several times did not make a complete recovery with the last. But the 3 additional years I spent nursing them provides additional protection vs. breast cancer, don't think I'd want to give up their half of that. And I have literally felt better and healthier in the years after 40, so rather have more of those.