Don’t they say teenagers/adolescence were invented in the 50s as that was the first time people were able to afford to allow their kids to carry on education?

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.

Maybe 23 would be a better average, but even if wvery women in your line gave birth at 12.5 that only doubles the other. And its fair to say not every mother would have been a first child. Also many still would have been born later than 25, so it probably evens out pretty well.

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

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.