[dilanj]

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 :)

[MikeUt]

I agree it's possible, but the developed world already has sub-replacement fertility. Each additional hurdle (freezing eggs, committing to fitness, avoiding other health issues, still having the desire for more children in your 50s, ...) filters away another significant fraction of the population, until what is left has no hope of replacement fertility, or even a manageably slow decline.

You'll hit a population cliff, with all the pain associated with that metaphor.

[dilanj]

You are absolutely correct. However as a individual decision maker your options are far limited. We should certainly think about economic incentives to address what you point out.