ダイキさん's Manga

I just don't get why MC has to be so young. If you think that it's fine because it's a historical, no it's not. Believe it or not, in no time in history has 9 years old been a normal (or even acceptable) age to marry. I've been told that MC has been aged up 2 years from the novel, and one of the knights' comment about how she should already have 3 children by age 18 is nonsense for ANY time period, let alone that in the novel he would've said this but for by the time she was 16 (3 kids at 16??? So what she'd have to start popping babies by 13??? no breaks???) I just don't get why the characters couldn't have been aged up to adults the whole time. It doesn't change the story other than some minor plot points (which could be solved if she were just some very naive and sheltered girl who got arranged married at 18yo with no knowledge of the outside world, it happens), and it just ends up being a really uncomfortable part of the story. It's not even about the age gap, it just feels like creepy af :/