And the fact that people in our old ages really did get married at a young as age (it still happens tho)
For me, the unexpected part about the age was actually due how the novel's synop was portrayed at the beginning of ch1. It seemed like everything in the novel that was described from marriage onward ranged around the mid teens but by the end of it, the reality is that they're actually on the cusp of puberty. This adds an extra layer of originality as it will include their coming of age and growth to adulthood.
If you put it that way, it makes more sense
(⊙o⊙)
The MC is what i'd expect of a person who "reincarnates"into a novel. Her shock, her despair. I suppose some people are just more adaptable to situations which seems to run amok in the MCs in most of their isekai and transmigration mangas. They somehow make the most of their situations w/ no problem most of the time as though the authors just want to get that part of the exposition over with. Her reaction here as displayed has aroused my interest in the story; not to mention ch 1 unexpectedly show them as 12 yo and not someone who is; say 16 yo.