I had my child at 41 (I'm the mother), and I see none of these issues, and my son has been over 10kg for quite some months now. My early-30s colleague on the other hand complained of back pain already in her pregnancy.
Age is not the death sentence you seem to think it is; on the contrary, if someone in early 40s doesn't have the energy/fitness to handle a child, they should see a doctor.
(I also got pregnant within a couple of months of trying, as did a neighbour of similar age. Personal biology trumps statistics.)
I'm not saying it's a "death sentence", just that physically, your body tends to deteriorate with age. The situation isn't a binary one: being young, your body is much more forgiving, which eases parenting, especially if you want to have more than one child.
So yeah, it's better to start having children early than late. It doesn't mean that you'll be crippled in bed because you had them late (aside from the fertility issues).
Also, Hacker News is essentially composed of well-off, western people working in white collar jobs. Of course, those people will have easy and healthy lives; I wouldn't say the same for a mason or a cashier, and those people exist.
Age is not the death sentence you seem to think it is; on the contrary, if someone in early 40s doesn't have the energy/fitness to handle a child, they should see a doctor.
(I also got pregnant within a couple of months of trying, as did a neighbour of similar age. Personal biology trumps statistics.)