What you're saying is immaterial because pre-pubescent traits are not what indicates whether a girl is ready for pregnancy or not; menarch is. Menstuation only comes when secondary sexual characteristics become evident, and that's what makes men sexually attracted to women.
And you're wrong about age of first pregnancy in the Ancient times being younger due to lifespans being shorter. The ideal age, physiologically, for a girl to get pregnant for the first time truly is 13-16, and not the abnormal 25+ years old that women nowadays get pregnant for the first time. The evidence? Fertility decreases by 5% per year after age 21 in women, to the point where a 31 year old has a 50% lower cance of getting preganant than a 21 year-old. By the age of 40, most womena re sterile and the ones who get preganant give birth to children with an unusually high rate of autism, schizofrenia, Down's Syndrome and epilepsy. 
SUCKMYMUSCLE
What do the outside effects of hormones in food, extremely high protein diet, and mdeical advancements have to contribute? For instance, back in thdays of King James, women had 15 inch ( and some 13 ich) waists. Not now. Why because women and men were smaller. This was especially noted when EUropeans came to America, where the Native Americans were noticeably taller (around the 6 foot mark), due to their high protein diets. Just go into a colonial house and see where the door frame is.
It is not uncommon for girls today to get their menses at 9 and 8 years old, even having "speed bumps" at that age. As a matter of fact, is it s a growing trend that girls are having this happen. This has probably been brought on by all the hormones in our food (ever heard of the song "Similac Child"). So to say that it is natural for a girl to have sex because of her menses is inconclusive, because most of of Western society relys on un-natural means to cultivate and grow food, and having un-natural diets as well as medicinal supplements within our diet that others before other did not have.
As far as those diseases being common in women who are apparent within their 40's, their is no finite evidence that suggests that, because, one, how do we know that those people were not genetically pre-disposed for those ailments in the first place. And two, how do we know that certain medications that were given to the mother or child (for instance the uproar over the increase of autism suppposedly due to vaccinations given to infants). Too many questions to rule out...
In order to say what is natural, we must first eat naturally and try to medicate ourselves naturally and then let nature take it's course.