Actually mens balls (testicle and scrotum) start sagging as soon as the testicles drop, which is just before he is born. Actually they wouldn't sag in the womb JK HAHA. You get what I mean. Males have sperm in the testicles, well sperm has to be at 94 degrees F to form. Since 98.6 F on average is the humans body temp., the body needs to fix the problem. It regulates this temperature by sagging or going up aka loosen or contract. Usually they will sag if it's warm and you are wearing loose briefs or boxers. They will contact for numerous reasons, being frightened, being active like running, and the most important, being cold. Chicks probably don't know this unless their man told them but it can get so cold that nuts go in the body. Not really far, but just enough to get warm, maybe 1/2 to 1 inch in. I surf, one time it was really freaking cold. I got on my board to paddle for a wave and, well, I will just say it hurt really bad to have my stomach on that board. Ouch. So, to answer your question age has nothing to do with this, I understand where your coming from though, "well I got boobs and I know they sag overtime...." but no they are totally different. The scrotum is like a bag that has muscles on it. They are very small muscles and you can't control them, kinda like your heart or guts. And they are constantly moving slowly and doing their job of regulating temperature. I hope I answered you question well, and taught you a thing or two about male anatomy.
Because of gravity.
My guess is that EVERYTHING sags after a certain age. Balls are no exception. Think of them as our breasts.