And setting the legal age of adult hood to 18 to 20 doesn't mean the human female body needs to reach that age to safely develop and give birth to a baby. The age of "adulthood/majority" is an artificially determined number based more on legal and political factors than any biological truth.
16 is like, the human civilizational norm, with weird outlier savages (bangladesh, aborigines, frontier Americans), doing 12-under, and then also odd Northwestern Euros sometimes hitting 20.
The truth though is, with good nutrition and a reasonable economy, starting a family at 16 for a man or a woman is fine and physically sustainable. Mind you, for Japan, the historical age of marriage is around 14. Frontier America was about 12-15 at times.
And for royals, whose marriages and kids could strengthen desired alliances and maybe even stop potential wars (like here), no one waited for the age of "adulthood", they wanted it to happen as soon as biology permitted.
Our
Queen Jadwiga, who got the crown of Poland at 10 year old when her father died, so couldn't be strong ruler on her own, but whoever would be chosen by nobles as her husband would instantly become king, got married at 12 with way older Grand Duke of Lithuania (strong and desired alliance leading to union of two countries) and had first pregnancy at 15. She was initially fiance of 3 year older than her
prince William, and when many Polish nobles started to turn away from that marriage, his father started to really hurry and got special medieval document from Archbishop testifying that Jadwiga was "precocious" and is allowed to marry William before her 12 birthday, medieval minimum age according to canon law.
Jadwiga obviously preferred the barely older guy and her first fiance, William, and according to some sources, managed to at least bed him when he was visiting trying to press his case, but Polish nobles chased him away. Here are
two paintings based on that
story of twelve(!) year old queen, trying to break gates with a fucking axe to get her lover back, while scared nobles try to hold her back.