Hey, getrealcut....I'll answer your email here as well. Generally speaking, there is going to be a difference anytime you increase GH by 2iu a day, considering that our normal production naturally is under 2iu a day once we are done growing.
I think younger guys need to use more than old guys to see good results because we produce 60% less GH by the time we reach late middle ages. The question for me would be: Should a 21 year old use GH at all? My son, who is about your age is still growing taller and putting on muscle mass naturally, so I would say no in his case.
If you are a serious competing BB'er, then maybe yes.
Everyone seems to react to exogenous GH differently. It depends on your individual rebound effect to the supplementation, your body's defense mechanism efficiency, and primarily, what you stack with it.
I have best results when using for 12 weeks - 4.5 iu the 1st month, 6iu the second, and 9 iu the third......insulin, an oral steroid, and plenty of test are essential for best results.