Don't forget that the models are usually pre-built with age parameters so the younger looking characters are usually built smaller by design.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.
Is it needed? No but it does convey as age difference and sure doesn't hurt the visuals from my standpoint.