chances are that at 19 you havent completely stopped growing from puberty- so you may already be naturally high in HGH and compounding it with the extra you have taken give you superphysical levels. (10+10=20)

older men who already have depleted levels of hgh, merely add hgh to a low base and they may work slower due to the initial lack of hgh. (2+10=12)

Using it when depleted (older) means less excess hgh- thats why the older guys say to wait.