Zygomatic arch broken 18 days ago, is it too late for a surgery? (Photo)

Isolated zygomatic arch fractures can be reduced through a
Keen (through the mouth) or Gilles (through the scalp) approach. As the bone
heals, the bone becomes more difficult to reduce (put back in place);
therefore, the bone should be reduced as soon as practical. The long-term
success of the reduction is dependent on multiple factors. Often the reduction
is unstable (does not stay reduced) because of the action of the muscles on the
bone and / or because of the multiple bone fragments. In these situations,
patients may have to either accept a slight bony deformity (depression) or
consent to reduction and internal fixation through a more invasive (coronal)
approach. A detailed examination and review of a CT scan of the face
will help delineate the best surgical option. Consultation with a plastic
surgeon would be the next best step.

it is not necessary to fix all facial fractures. Surgical intervention is determined by several factors. If you have no facial deformity and/or painful mouth movements like chewing or malalignment of your teeth, then there's no indication to fix the fracture.