Well, you would start to put on your best gains from now until 23-25. I know you probably wont wait till your 25 if you already have the itch to cycle. But do yourself A favor and wait a year or preferably 2 at the very least.
Take that time to critique your training , diet, and techniques. Gear is not a miracle drug. The guys that get lasting and worth while gains when on, know how to make the same gains off. So put on a few pounds and find what works for you. And if you neglect all the advice then you need a serm for pct and should have an ai on hand in case of sides (gyno) arise. And I don't mean any OTC Ai's