The numbers go way up in the relative sense but are still quite small in the absolute sense. Evolutionarily speaking I think the differences are irrelevant.

Evolutionarily speaking, the dangers of having older parents are definitely dwarfed by the fact that older parents just tend to have fewer kids.

Cultures that encourage starting families at 25 will genetically dominate cultures that encourage starting families at 35.

Even though I agree with you, I would like to point out that more experienced and stable parents could be enough of an advantage to balance the benefits of having younger parents. Especially for fathers that have already a settled career and more time to invest in their offspring.

If it were as simple as DNA errors accumulating with age we should observe 1) big population-level jumps in complications when parents started having babies in their 20s rather than teens (in India, my grandparent's generation, 14,15 for the mother and 17,18 for the father) and 2) big population-level effects in complications across birth order. Do we?

Isn't it strange that human cultures at some point thought "teenagers" are when people should start families all over the world from the rolling hills of Bahia to Tamil Nadu to Guandong to the riverlands of Ahafo?

Thats more than simple correlation. Having kids when you are in middle age might have been known as physically difficult, especially for the mother. We've made huge leaps in medicine to trivialize child birth but for most of human existence, it was a game of dice, to some degree it still is, where mother and/or child could perish.

Maybe younger women cope better with the physical stresses of pregnancy? Nature seems to think so.

> Isn't it strange that human cultures at some point thought "teenagers" are when people should start families

Not really. That's when they start having sex. So you end up having kids whether you like it or not.

It isnt so simple as they start having sex. The idea that teenagers sleep around like rabbits is relatively new. Usually sex followed some form of commitment - this case being marriage - in order to protect young women from free willies. Sex wasn't always free.