There is no doubt that a couple of centuries ago girls were (mentally)adults by 12, 13 and 14 years old. When you were most likely dead at 40, 13 was adult. :P

However there were reasons so many women died in childbirth then. At 12,13 or 14 a women's body, despite having reached puberty, is not physically ready to carry and birth a child. The chances of long term physical problems are too great.