I donm't think 11 yos are ready for sexual contact obviously!! But the point I am making is that in a primitive society , and in the animal world females would be getting pregnant as soon as they were sexually mature, and presumably most survive
No. Not most. Enough survive. Enough for evolution not to keep on evolving so birth isn't so inherently risky for females and babies in that species.
Girls (the human kind) under 15 are twenty times more likely to die during pregnancy and birth than women over 20. Their children are at a higher risk of prematurity, stillbirth and being part of the infant mortality stats as well as the perinatal mortality stats.
To give context, birth even in countries with low perinatal mortality such as the states is 14 higher than abortion in the same country. Birth has always been dangerous and potentially deadly. The more risk factors added on (and precocious pregnancy is a high risk factor) the greater the potential for injury and death.
And survive does not necessarily mean a good outcome in the physical (let alone the emotional) sense. For an example of just one post birth complication, the rate of fistula leading to shunning and increased degrees of poverty is high in cultures with poor obstetric care. The rate and degree of damage is higher still in younger mothers. I cried buckets at the waste of female humanity as the result of fistula alone after watching a documentary. They were alive. But, whole they were not. Not physically, not emotionally and not psychologically. Being a pariah is no way to live.
Very many of the girls on the list of known very young mother suffered from precocious puberty, an anomaly, with menarche as young as the first year of life and breast development in toddlerhood. Something that tends to get treated these days where quality medical care allows for it.
Nature is not some wise, fluffy entity that knows best. Nature does not care. As long as enough (not most, not many, just enough) of us survive, regardless of state of wellbeing and intactness, then that is "good enough" for nature.
And anybody who remains unconvinced of that needs to google birth in hyaenas and then come back and see if they still think nature is in anyway invested in "most" of any species of mothers or babies surviving for their own sake.