Muscle memory and habit are the culprits for Adenoid face. When the mouth is forced to be open consistently, the facial muscles will develop around that posture. Once they are developed, they will adapt and function improperly to compensate for the open mouth posture.
Children will grow and develop around the fact that they cannot breathe through their noses properly. It will change the way some of the most basic functions work, such as chewing and swallowing food, drinking liquids, forming words, and swallowing their own saliva.
The mouth breathing may just start as a habit, but if it’s not addressed these facial changes will remain into adult years. Growth is a powerful tool for change, whether good or bad. A child with an open mouth will grow into an adult with the same features and difficulties that come from having an open mouth.