Gratian, the man who founded Canon law placed the age of consent at 7, marriage at 12. His view on law affected the entirety of Europe and later America. The UK had an age of consent as 10 until 1875. It was considered absolutely normal and socio-economic reasons are part of it, which you attempt to contradict. Another part of it was the Church, Papal law didn't have an age of consent, only an age at which sanctity of marriage could be announced, and that was age 12. This was due to the belief that Mary was 12 or 13 when she gave birth to Jesus.
The mother of Henry the 7th was 9 at her first marriage, 12 when she gave birth to Henry (as part of her 2nd marriage).
Your point about the danger and lack of medical science is the opposite of what was going on. It was necessitated that girls had children as young as possible due to life expectancy and infant mortality rates, the younger you started the more likely you were to succeed in the long term. The majority of upper class children were born to mothers between 13 and 18, this being due to political pressures of producing heirs.
The idea that young mothers is a bad thing is a fairly recent moral construct maybe within the last 100-150 years, which is hilarious considering that precocious puberty is on the rise, we are trying to have children later and our bodies are adapting to have them earlier.
History completely disagrees with you, as far it is concerned young mothers were a good thing until we decided as a society that it wasn't.