This question is not on the morality of marrying or having intercourse with what we consider today a minor, as I have no issue with that especially when framed historically and geographically, rather my issue is that of the increased maternal mortality for adolescence and children.
We are well aware that women also commonly gave birth around that age;
Ash-Shaafa‘i also said: In San‘aa’ I saw a grandmother who was twenty-one years old; she reached puberty at the age of nine and gave birth at the age of ten, and her daughter reached puberty at the age of nine and gave birth at the age of ten. As-Sunan al-Kubra by al-Bayhaqi (1/319)
However today, we know that the maternal mortality rate for adolescent and child mothers is up to 2x larger in the case of 16-19 year olds and up to 4x larger in those younger.
mortality rates of children born to mothers aged <16 years, 16–17 years and 18–19 years at first birth were about 2–4 times, 1.5–2 times and 1.2–1.5 times higher, respectively, than among firstborn children of mothers aged 23–25 https://gh.bmj.com/content/7/5/e007681
This study was also conducted in sub-saharan Africa where we know that the age of puberty for women (8-13) is similar to the middle east now and during the prophetﷺ's time. The study also considers absolute mortality decreasing over time but says that the gradient between ages has remained the same so it is not related to past or new medical advances but rather the physical capability of the mother.
As such, why was intercourse allowed in consideration of a young person's higher chance of mortality? This is not to argue the morality of the age specifically but rather the effects of child birth on those women - as Allah is all-knowing would he not have banned or suggested coitus interruptus (or any other method) till later in the wife's life?
I don't mean that statement as a jab, of course, I was just wondering what the hikmah behind child marriages and young births was considering the risk involved. I've also heard of the average life expectancy argument however as far as I know, it was quite low due to a high infant mortality rate and that once an age threshold was reached, the life expectancy would increase to up to even 50+. Do correct me if I'm wrong though Jzk