Is 18 too young for brow lift? The eyebrows droop in front of my eyes, making me always look very cross, angry, and tired.

Many options are available to you from neurotoxins such as Xeomin and Botox and several browlift options.  I would suggest consulting a board certified plastic surgeon to determine your best options.

The indication for a brow lift is true brow ptosis. Like rhinoplasty, the patient  should have completed their growth spurt before elective cosmetic brow lift surgery

Without photos, it is hard to be more specific, but in general terms, there are a few, simple nonsurgical approaches, which include the injection of microndroplets of neuromodulators, such as Botox, Dysport or Xeomen, either alone or in combination with volumizing fillers injected finessefully in particular locations above and below the brow (The Nonsurgical 3D Vectoring Browlift) that can restore lift and proper projection to droopy eyebrows. I have found this combination approach especially helpful for younger persons with premature drooping and sagging of the brow, which seems to be the case here. The entire treatment usually takes no more than fifteen minutes to perform and engenders little to no downtime.  For more information on the above, I suggest you check out the archives of Realself.com.