Sweets, at 18 you will NOT get saggy skin. I did a 33-day wf at 28 y/o and I had no saggy skin. Saggy skin-wise you will be *FINE* I will give you a signed promise note :)
How much weight you will lose does not depend on the fast. It depends on what will happen in the first 3 months *after* the fast. Post-fast binges are no joke. Have a solid plan and a support system. I regained all weight I lost on the 33 days, even though I completed the 33 days. The fast is not the main event, THE REFEEDING is the main event, and not just the immediate 2/3 days.
If your goal is weight loss, WF is not the best way to go about it. juicing is (don't forget vit B12 and Omega3!). You can sustain juicing longer, you still get vitamins and minerals, you can exercise and continue to build strength while you juice, and the refeeding is easier.