Adolescence and early adulthood are when testosterone levels generally peak, and after the age of 30 or 40 they decline by about 1% a year. Low testosterone levels in older men are usually due to the normal aging process, but it's important to determine if this is actually the cause, and not due to hypogonadism, a disease that hampers the ability to produce normal amounts of testosterone due to a problem with the testicles or the pituitary gland that regulates them. Lower testosterone levels, and the resultant effects, can be disconcerting but they are a natural part of aging; if you are particularly concerned, however, speak with your doctor. They will likely measure your testosterone levels at least twice before recommending testosterone therapy.