What's the cause of a droopy eyebrow at 19?

Dear sarah.anderson,Browlift
or a forehead lift is done to decrease the amount of lines and wrinkles on the
forehead. The browlift will reposition the brow and get rid of any sagging.
Brow lift and forehead lift also act to smooth horizontal forehead lines and
the ‘vertical 11’s’ between the brows.I suggest you consult a board certified facial plastic surgeon to receive the best in-person treatment, care, knowledge and surgical skills available. The consultation would allow the surgeon to closely examine your eyebrows and determine the potential causes and potential remedies to the problems you are describing here.

It is possible to have typical causes of eyebrow ptosis at any age. Facial asymmetry is likely the underlying cause, but other factors should be excluded prior to treatment. An examination should be able to asses and make sure that facial nerve weakness, eyelid ptosis, or other neurological conditions do not underly your issue. Provided that is the case, a simple upper eyelid lift should be able to help you regain symmetry.

True brow falling can occur in any one. True brow ptosis most commonly occurs as people get older. At your age, if you have true brow ptosis it is most probably a familial or congenital condition. Other rarer conditions like trauma or facial nerve injury can also produce this look. Therefore it is very important for you to have an accurate diagnosis before any treatment is planned.

I would recommend first looking closer at other old photos to see if this is a really new change or something that has been going on for quite some time.  Almost everyone has some facial asymmetry with one side of your face being the longer and taller side and the other being the shorter and and wider side.  This can manifest by one eye, eyelid, or eyebrow appearing more prominent that than the other side.  As you have already done some of those, I would schedule with oculoplastic/orbital surgeon to review the photos with you to get a full orbital and eye examination and determine if any further testing is necessary.