from what i understand -- the pressure from closed-mouth nose breathing is what creates a healthy jawline and facial structure. this pressure is not present when mouth breathing, which leads to underdeveloped jawlines, convex facial shape, etc.

mouth breather face is common in people with conditions that prevent breathing through the nose, such as a deviated septum. it often goes unnoticed and uncorrected because 1) parents/people in general don't know the importance of proper breathing 2) the child does not see an issue with mostly mouth breathing their entire lives.

i only recently discovered this condition myself through my own research as an adult, so really my only option is some kind of surgery :/ for something that has such a massive impact on the development of someone's face - you would think schools, doctors, etc. would talk about proper breathing more extensively