Not even that far back, modern medicine is wonderful

Many people don't realize that the amount of change our culture goes through in a lifetime is unfathomable historically. Before the 1800s it took a good decade for news to truly travel around to everyone in a region, and that was considered timely if it happened at all. Farming, hunting, homemaking, war, stayed exactly the same for dozens of years at a time and changes were usually made abruptly due to conflict before stagnating again.

I am interested in learning about this metric time.

What is the conversion from imperial mother to metric mother? About 1:1.26?

And if your grandmother had wheels she would be a bicycle.

This assumes a single child per set of parents, doesn't it?

Humanity isn't progressing uniformly forward like this. Lgbtqia+ people were considered normal part of society by various cultures. Also Magnus Hirschfeld was an advocate for lgbtqia+ people a hundred years ago. Slavery has been transformed into modern slavery because the western world has found other, more concealed ways to force people into labor. Ideas may die out, but they will pop into people's head again and again.

Two steps forward. One step back

That's also assuming you're the first born of the first born of the first born, and so on. And the further back you go, the more individual kids the average mother is likely to have. After all, you had to have like 12 kids just so 3 of them would make it past 9.

So your greatx12 grandmother might've started having kids at 15, but she still might not have had your ancestor till years later.

You would have a lot more death during pregnancy / childbirth though.

As the other commentator says, medieval Europe was mostly early twenties. Studies of stone age remains suggest a first birth age average of 19.5 and contemporary hunter gather societies have a comparable average. Sexual activity generally begins earlier, during adolescence, but the most "reproductively successful" age for beginning childbearing has been shown to be around 18-19. Also, this age at first birth isnt "Average age of a child's mother" as many women would have multiple kids over their life, so the average sibling would have a much older mother at birth than the firstborn.

Its important to remember that puberty has shifted massively since industrialisation, "menarche age has receded from 16.5 years in 1880 to the current 12.5 years in western societies". So the post-puberty fecundity peak, that use to happen 17-19, when women are fully grown enough to minimise birth complications, now happens at a disressingly young 13-15. Not only is this a big social yuck for most western societies, but it's reproductively unideal, because of the complications linked to childbirth at that age.

In Western Europe at least back to the early medieval period it was common for anyone who wasn’t nobility to have their first child around 22. The younger you are the more likely you’re going to have serious (fatal, back then) complications. It was the nobility that was marrying off barely pubescent kids.

First births yes, but what about average age? Our ancestors may have been second born, third born, eighth born etc