You already know the sides associated with
test...you can be ready to combat some of the sides with ancillary drugs.21 is pushing it but there are many who have done this already.I have a good friend who is 22 and trained for 4 years 5' 11'' 220 he just ran 1st cycle and gained 10 pounds quality mass.My advice is keep training and try to really consume an enormous amount of cals over the next year..if you gain 5-10 pounds from that I would do it another year and keep doing it untill you really hit platue of not gaining..then run
test enth or
cyp for say 500 mgs per wk for 8-10 and have proper pct lined up.Buying the gear and getting legit meds will be your largest obstacle....good luck