It was like that before (and still in some countries), as soon as a woman had her first period she was seen in age to have children, so she could have sex.
In some cases the first time they had to do it front of witnesses, like that they knew the act was done and if the woman was a virgin seeing the blood on bed (in fact women's bodies and the himen don't work like that)!
That is why in some cultures, as soon as a girl have her first period she needs to cover her head or be adorned in certain way to indicate she was fertile and in age to have children, some of these cultures still practice adorning/covering women as soon as they have theie period but not with the same meaning, the meaning is that they are adults!
So yeap, it was normal 12 years old to marry (many times younger) and have sex as soon the girl have her first period!
Note that some girls have their peeiod as young as 10 years old... so yeap, women had to fight to stop some stuff like this, sadly there are still men who defend this practice, there are thousands minors being in forced marriage in the USA every year and many occidental countries still happen, even in Christian/Catholic communities!
that's all well and not dandy but I don't really see the purpose of having historically accurate behaviour (?) in a fantasy setting that btw doesn't have any real life locations, religions or cultures
like, this story has tiny little spider demons and floating chibis so could we possibly not have the twelve year olds get pregnant? please?(〜 ̄△ ̄)〜
if it's sixteen I'll tolerate it a smidge more, but not like it, but just....why?
I know I went on a rammable but I get the setting the author made and I still like the story, guess I just hated the image of a pregnant 12/14 year old the author placed in my mind
for her time period her age is the right one for engagement not marriage it is when she has her coming of age that talks of marriage like the whole wedding can begin. here the guy is marring her to protect her from her wicked family, just being the younger sister of that blonde is not enough since her wicked family has some prestige for their work so marrying the former king would offer her better protection from there, the guy isn't going to sleep with her, for fucks sake he had a kid her age and knows how to take care of them. the girl her self looks too small for her age because she was underfed and maltreated so it looks like he's marrying a 12 year old but i think she is older than that
I think it's more:
Engaged at 12 years.
Married at 16/18 years [when he became of age].
Child Amnat: 2/3 years old.. so if the wife died a couple years after that (3?)...
King = 22/24 years old (?)
Second wife went yandere ... 1-2 years in?
So he's maybe 24/26 at that time?
Then his current age is: 25/27? [Though he also could be way older because that goatee makes him look ancient.] I suppose we'll find out when we see how old the current king is (his kid). You're right though. This age-gap is doing my head in.

....did they make twelve year olds fuck???? I mean in the last panel on the first page he looks taller then her so I assume maybe a year or two past but....did they make fourteen year olds fuck????
no wonder this age gap thing is draining my sanity