> The dreaded issue of the 'biological clock' is where cryopreservation - or egg freezing - has completely changed the game. Straight from the intro of Idiocracy. And say you have your 1st child in your 40s. What about 2nd and 3rd? It takes 2.1 children per woman to sustain a population. How many 50 year old parents with a 5 year old running around will have the energy for a second child?
As I point out, your biological ability to have children doesn't diminish. So I don't see why you can't have your 2 or 3 children. There are of course options like surrogates, and other more cutting edge tech enabled options. And as for energy, the point is to commit to your fitness so that you'll be a very healthy and energetic 50 year old :)
You'll hit a population cliff, with all the pain associated with that metaphor.