The birth rates are plummeting because people are taking longer to start their lives. Women are in school until 23 and tend to change partners at 26. This means there is a 9 year window of being marriable for children until 35 at which fertility dramatically declines. I'm talking about five trips to an IVF clinic or 75% chance of conception using donor eggs for a just a single child.
I don't want to see this too negatively though. In principle the pool of women who stay fertile longer will outproduce women who lose their fertility too early. It also means there is genetic selection for longer lifespans in general.
As someone who knocked up their wife, who is well over 35, this year, I think your understanding of the drastic drop off in fertility at that age is exaggerated.
All of what you said I do expect to be the expectation past 40 though.
I don't want to see this too negatively though. In principle the pool of women who stay fertile longer will outproduce women who lose their fertility too early. It also means there is genetic selection for longer lifespans in general.