You don't need to use SD at 21. First look at your diet and training to see if there is some improvements to be made. You start jumpin on gear (yes SD is a steroid, and a very potent one at that) without the essentials in check you are just askin for the yo-yo ride from hell. Wow! I gained 15lbs on SD! Oh crap, I lost it all within a month.

Better do another cycle! Wow! I gained 10lbs this time! Crap, lost it all again.
Been there, it sucks.