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.