Memes

well ignoring how you moved the goalposts there from 26 to 40 there's a lot of misleading information here

by 40 half of women are completely sterile (even though they are a decade away from menopause).

I'm not sure where you got this but it is untrue. The chance of a couple getting pregnant each month they try is 20-25%. by 35 this has dropped to about 15% and 5% from 40 until menopause. This means over a year of trying your chances go from 95% to 45%, that does not mean 55% are infertile, it just means they didn't get pregnant in 1 year (also keep in mind this is accounting for the fertility of BOTH people in the couple, not just the women like you seem to be suggesting).

Not to mention that people have less energy to raise a child as they get older, and complications like autism or down’s syndrome spike after 35.

There's quite a lot of research on this shows that the age of parents has little to no correlation with health or wellbeing outcomes in the child, but large positive effects on the health and wellbeing of the parents. for example . While their are some health complications that increase past 35 there are also many that decrease. Older mothers lead to a slight increase in birth defects, lower birth weights, older fathers are linked with higher incidence of autism. However, a study of 56K children shows parents under 25 have worse health outcomes in terms of height, obesity, self-rated health, and diagnosed health conditions. The finding of that study seem to match all the others I find, that the Idea age cohort for best overall health seems to be 25-34 for pretty much all outcomes.

Here is an anecdote to justify that data: my mom was 45 when I was born, (funnily enough the misinformation around fertility directly lead to my existence) I had a much better childhood than my brothers. They had far more energy for me because they were further on in their careers, retired when I was a teen, had far more money, and had the maturity to quit many of the bad habits they had in their 20s.

Keep in mind that all the effects are pretty tiny though (take a look at the y axis on the graphs of the source I linked earlier), I think you'd be pretty silly to even have this in your top 10 for decision making, most governments seem to agree. Your socioeconomic status, smoking/drinking status (even if you don't while actively pregnant), how close you live to a highway all have far bigger impacts on the health of potential children. If not getting pregnant would be extremely distressing then yeah, probably best to get started early but otherwise there isn't really a scientific grounding in the idea we should be having children young.