In Ancient Egypt, mid twenties was old age (unless you were part of the ruling class). So, until what age could women still bear children? I would think a 20 year old pregnant woman would be kind of like a present day 40 or 50 year old. It could happen, but not often. So, people married and started families 'early' ... by our standards at least. Pretty much, the instant a girl entered puberty, she was marriable. Twelve, married and pregnant would not have raised any eyebrows back then.
Methinks that even in Ancient Rome, 12 was the acceptable age to marry, for girls anyway, iirc, boys were supposed to be capable of taking care of the family ... ie, adult, ie, 16.
From what I gather from the Homeros, in Ancient Greece, girls/women were pretty much property, just like in the bible. Fathers handed them off to gain advantage somewhere, lords assigned women to faithfull servants and women were counted among the loot after a raid. In fact, the reason Achilles was pissed at Agamemnon was that the latter had nicked the former's share of the loot, ie, Briseis.
But what all of these have in common is puberty. (or at least a standardized age when puberty was supposed to have started) Before puberty, hands off! Which makes sense from an empathy point of view. Sub-teens, as Anthony calls them, aren't ready, physically and/or emotionally for sex. So, forcing them into sex hurts them and is thus a no-no.