Men experience age-related hormonal changes in a process that many physicians refer to as andropause. In men, this decrease in testosterone levels usually happens very gradually. Most men begin to have declining testosterone levels around age 30. Testosterone is produced by the testicles and is responsible for maintaining muscle mass, adequate levels of red blood cells, sense of well-being, motivation and drive, bone density, skin collagen and sexual and reproductive function. Other factors that may lead to lower testosterone levels include traumatic brain injuries, prior concussions and former anabolic steroid use.