What is the reasoning for 21+?
It can take ages to reach your genetic potential and why start to get big when your in your late 20s? why not be big in your prime when its gonna benefit you more.
I thought as long as your not fat and growth plates have closed and you have been training and dieting long enough(1-2 years with no breaks) to show yourself you got what it takes to not mess yourself(become fat on cycle, or stop training once stopping your cycle) then what else is there to it?
I dont believe there is a particular age, I think if your borderline on age then to go to a doctor and get your growth plates checked, which would be a great idea because you could also get your natural hormone readings.