Hi, i'm a guy i would like to know if chewing gum or teeth grinding could cause a square jaw and how long does it take 2 develop a square jaw 2tru dat?

Genetics: The size, shape, and position of your mandible is genetically predispositioned (unless you sucked your thumb excessively as a child which could have altered growth). You're not growing anymore at 19 years old. Chewing gum excessively can cause TMJ problems, and grinding your teeth will make them look smaller, collapse your vertical dimension, and make you look older.

Masseter hypertrophy: Clenching and grinding your teeth may increase size and appearance of masseter muscles and completely change facial appearance (square jaw). Botox is an excellent treatment option to soften, relax, and return facial shape to normal size.

Jaw shape: Jaw shape is genetically determined. Temporary soft-tissue architectural changes can be achieved with 12-14 hours/day exercise or by Botox injections. Permanent skeletal alteration, however, requires Orthognathic (jaw) surgery. For more information, see an Oral Surgeon.