Having adequate levels of testosterone is a vital factor in determining your quality of life. For beginners, testosterone is a male sex hormone found in men and women found to be important for energy, fertility, healthy sexual functions, and building muscle. Testosterone is one of the main causes of physical changes for boys during puberty, like promoting deeper voice and hair growth. Generally, a person’s testosterone level will steadily decline as you age, depending on medical conditions, lifestyle choices, and other factors. A decreasing T level can result in a lot of changes in your body, including a lower sex drive.