You mean saggy scrotum, that's the skin sack that holds the testicles and other parts inside. The testicles are the firm, oval balls inside it. The sagginess and firmness of the scrotum is related to the sagginess and firmness of the skin. With age the skin tends to sag and so is the scrotum which is made mostly of skin (the skin loses its elastic fibers which hold it tight together like elastic bands).
Testicles and scrotums are like finger prints, their all different.