Ah, it's the bitter commenter
@SotiCoto.
The number 40 wasn't really the important part, it was a random guesstimate but I guess I was wrong.
But you did address the main question "instincts and motivations should logically be those of a 10-year-old because they're dictated by the state of the body". That's what I've always thought! I mean, it's maybe more complex and blended than that but isekai always have MC's that are like, "can't be with these girls cause they are 1/4 of my age, fofofo" while in the body of a 12 year old or something.
There exists a "formula": age/2 + 7 is the minimum dating age (naturally a guideline, not a rule) but I feel like there'd be a diff equation for isekai MC's.
Anyhow, that's just a theory. A math theory.
As for "where the hell did you get 40 from?", probably from reading too many isekai manga in a short period of time. They are all just so same-same, they blend together.