Agree...
The reason why the MC in these games always looks so small is because it is really the only way to illustrate the younger older thing since there really is no skin or method available to show the age of the characters.
I don't honestly see it as a necessary thing in the case of this game, even much earlier on in development.
The facial structures and features of all the women, except Julie, manage to look older than Jason
I think that the age difference between mother/Janice and Jason & Julia/sister is actually shown pretty well in Janice's face.
Her face looks older without "looking old", looking more mature.
Kelly's face looks more mature than Jason/Julia but still younger than Janice, while Emma looks to be between Kelly and Janice, although that could be because of her skin colour.
So far, the only woman where the "shorter = younger" logic applies, is with the teacher, because she's the youngest looking of the adult women.
Even then, she still does look older than Jason, but (in my opinion) only by around 5 years at most.
Unfortunately, it's ultimately far too late in development to change the height differences, due to all the animations being rigged with the current models.