thats not what anyone is saying.
What everyone is saying is, right now, at 21, you have high natural testosterone levels. In fact they are peaking. In a few years, your natural test levels will begin to decrease; therefore, natural muscle building will not come as easily. That is when you use aas, to supplement/exceed your natural levels. If your body already has dense muscle tissue from years of natural training, the aas will blast you beyond whatever is natural.
If you use aas before your body is ready for them (ie. Already has high test, little dense muscle tissue with no memory) then when you come off, your body will go extremely catabolic because, basically, its saying to itself "what the fuck is going on! I cant support all this. Take it away." then you are left with dwindling natural test levels because you will never fully recover, and shitty body composition on the later years because at 30 you will still be running on low natural test while you are trying to build a solid natural body. That will lead to trt and, if you want any decent muscle at all, blast and cruise the rest of your life.