Paleolithic life expectancy was something like 22 to 33, and much of that after very high infant/maternal death rates. With such life expectancies, females would likely be bearing children at ages we today would find positively shocking. Simply surviving childbirth would be a huge selection factor for both mother and child. And girls become fertile well before they are fully developed physically (yes?), and I suppose their ability to survive childbirth presumably increases a lot with each year after menarche until they are fully grown.
This might be noticed and communicated culturally. There could certainly be selection advantages to parents and families communicating this to their daughters. On the other hand, adolescents certainly want to have sex at early ages, and our ancestors wouldn't have the same hangups and cultural barriers humans have had for the past several millenia. (Indeed, part of the story here is that developing those hangups is, to some extent, adaptive.). And with shorter lifespans, there would have been some expectation of getting on with it. Sex and mating would have been socially "valid" choices at far earlier ages.
If so a girl (/family?) with the intelligence and self-control to 1) notice this and 2) act to postpone mating, even by a year or two, could do a lot to improve a girl's/woman's odds of surviving childbirth and so passing on her genes. If (a big if?) we presume that these characteristics are related to brain size, that might explain it. (If the ability to notice, and communicate, the advantage of waiting were in the _parents_, that would still be be selective for intelligence/control.)
As human lifespans extended, pressure to reproduce at an early age would diminish, and it would be easier to create and communicate the cultural norms protecting girls from maternity before they were fully grown. Indeed those norms would become adaptive, as having sex too early would be needlessly dangerous. At which point, _all_ girls would be protected from premature pregnancy, and the selection for intelligence/control would diminish.
(Apologies for any ignorance of how the ability to bear children changes after menarche, I have only the standard male knowledge of these things. And for "girl"/"woman" nomenclature issues, the ages in question here are very different than the ones we are today accustomed to.)