Men produce the majority of their testosterone in their gonads (testicles), and a smaller amount in their adrenal glands. Testosterone is known as an androgen, a hormone that is responsible for many male characteristics such as facial hair, sex drive, and higher relative muscle mass than females. Testosterone levels normally fluctuate (sometimes literally fluctuating between night and day), and men typically produce about 7mg of testosterone in a 24-hour period. As you age, testosterone levels naturally begin to fall. Research shows that roughly 2 out of every 10 men over the age of 60 have low testosterone, or low T. That number jumps to 3 out of every 10 for men between the ages of 70-80.