Loose neck skin at 20, what are my options? (Photos)

It does NOT appear to me that you have loose neck skin from the photos.  NO surgery is indicated for a patient with a neck as nice as yours.  Having said that, if you would like to have a nonsurgical treatment over your younger years to keep the skin firm, then you might consider Ultherapy in your 20s and 30s to keep maintaining skin firmness.  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  I've attached a link where you can start your search. 

A combination approach will give the most improvement with botox, ultherapy, thermage, ThermiRF, and PDO threads.  I suggest you get a formal evaluation with a cosmetic dermatologist. Best, Dr. Emer.