The tightness of your scrotum varies according to temperature and sexual excitement. The first stage of puberty in boys involves growth of the scrotum which lowers the testicles. There's an important reason for this.
The ideal temperature for sperm production is about 1 degree cooler than normal body temperature. That's why your testicles are outside the body. If they become too warm a muscle in the spermatic cord relaxes and they hang lower, loosening the scrotal skin. If they become too cool the muscle tightens and pulls them closer to your body for warmth. (This is why they're usually pulled up close when you've been swimming).
So, a tight scrotum doesn't indicate anything about health. Some guys just have more scrotal skin than others.
They "drop" at two points in your life. First, the testicles develop up inside your abdomen before you're born. At some point they descend down through the inguinal canal and end up in the scrotum. (If this doesn't happen it's called an undescended testicle or cryptorchidism).
They stay fairly close to the body until puberty. In the first stage of puberty (Tanner Scale G2) the testicles increase in size and begin to "drop" lower from the body. This is in preparation for sperm production. (The whole temperature thing I talked about). Sometimes this is referred to as "dropping."
The older a guy gets the more he loses elasticity in the scrotal skin so they will almost always hang lower.
Hope I answered your question. If you have any others feel free to post again.
I don't necessarily have an answer, I just wanted to add a observation that I had this morning. First, one of my testicles became sore last night. During the night I noticed that it had become very tender when I rolled over, which it made it difficult to fall back asleep. In the morning it was quite sore so I gave myself a physical examine. When I did this I noticed that one testicle was very swollen, and my scrotum was hanging low. As I know that this condition could possibly be testicular torsion I stood with my legs open and wiggled in hopes that if I am suffering from torsion the testicle would have a chance to rearranged itself to a healthier position. An hour later I tried to do this again, however, I noticed that my scrotum was tight, and that this would likely not be possible now. This made me curious to a question I think of occasionally about why my scrotum hangs sometimes when it is usually tight - at least for me.
My comment is that my experience today lends some credibility to PaulK9378's, and others, comment that it has to do with temperature. When I got out of bed, where I was warm, my scrotum was hanging low. Soon after, it was taut. I covered up again and waited to see if it loosened again, and it did.
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