Germ theory didn't exist, or at least wasn't accepted, until the late 19th century. In fact even things like handwashing before surgery weren't accepted until a similar timeline. Prior to that it would have been insulting to insinuate that a surgeons hand's might be 'unclean.' All the money in the world couldn't change this lack of knowledge. And indeed one of the Founding Fathers, George Washington, was likely killed more by the bleeding edge treatment of the day, than the disease that was being treated. In response to a throat infection he was bled for nearly 5 pints in order to remove 'bad blood.' His deathbed portraits showed him as white as a ghost.

You're right we don't need to have as many children per person thanks to reduced infant mortality. Each and every woman having an average of 2.1 children is all that's required for a stable population. So each family that can/does have children should probably have somewhere between 2 and 5 to compensate for those who will not or cannot.

And indeed we do need to start relatively early. By the time a woman reaches her 40s her fertility (assuming a perfectly timed effort) is going to be around 5-10% per month. When she's younger that can be > 30%. And in between each child you really want at least 9 months before starting on the next, a bit more is even better. And then you need to actually succeed at pregnancy. Contrary to popular conception things like IVF are not just guaranteed pregnancy. They're an extremely expensive roll of the dice. The dice are more weighted in your favor, but the odds of success can still be quite low for older parents. So perhaps 2-3 years per child, increasing with age, multiplied by 2-5 children. That's a lot of years and you want to finish this all up before your 40s if possible.

And most importantly of all - this isn't an option. Societies that fail to maintain themselves will simply die off and end up being replaced. Much of what I'm saying here runs face first into contemporary ideological ideals, but the reality of what I'm saying will win simply because contemporary ideals are not self sustaining.