Brows sag as well as other facial and body structures, due to reduction in skin support or laxity as well as gravity.
Your brow is contains skin, muscle and fascia holding it to the skull.
In addition, eyelid position is somewhat determied by the brow, so eyebrow is elevated as well.
Brow surgery, browplasty or brow lift is performed via scalp incisions either endoscopic or via a coronal incision.
Endoscopic Browlift Video (click in box to load)