It is not clear whether aging itself or the disorders associated with aging cause the gradual changes that occur in men's sexual functioning. The frequency, duration, and rigidity of erections gradually decline as men age (see Erectile Dysfunction Erectile Dysfunction (ED) Erectile dysfunction (ED) is the inability to attain or sustain an erection satisfactory for sexual intercourse. (See also Overview of Sexual Dysfunction in Men.) Every man occasionally has... read more ). Levels of the male sex hormone (testosterone) tend to decrease, reducing sex drive Decreased Libido in Men Decreased libido is a reduction in sex drive. Possible causes include psychologic factors (such as depression, anxiety, or relationship problems), drugs, and low blood levels of testosterone... read more (libido). Blood flow to the penis decreases. Other changes include
Decrease in sensitivity of the penis
Decrease in volume of fluid released during ejaculation
Reduced forewarning of ejaculation
Orgasm without ejaculation
After orgasm, penis becomes limp (detumescent) more quickly
After orgasm, longer period before an erection can occur (refractory period)
Beginning at about age 20, the production of testosterone (the main male sex hormone) in men usually begins to decrease an average of 1 to 2% per year. The period later in life when testosterone production has decreased enough to cause significant symptoms is sometimes described as male menopause, or andropause. However, the gradual hormone decline in men differs greatly from what women experience in menopause Menopause Menopause is the permanent end of menstrual periods, ovulation, and fertility. For up to several years before and just after menopause, estrogen levels fluctuate widely, periods become irregular... read more 
, during which female hormones almost always decline rapidly over just a few years. The rate of testosterone decline varies greatly among men. Some men in their 70s have testosterone levels that match those of the average man in his 30s.
Whether young or old, men with low testosterone levels may develop certain characteristics associated with aging, including decreased libido Decreased Libido in Men Decreased libido is a reduction in sex drive. Possible causes include psychologic factors (such as depression, anxiety, or relationship problems), drugs, and low blood levels of testosterone... read more , decreased muscle mass, increased abdominal fat, thin bones that easily fracture (osteoporosis Osteoporosis Osteoporosis is a condition in which a decrease in the density of bones weakens the bones, making breaks (fractures) likely. Aging, estrogen deficiency, low vitamin D or calcium intake, and... read more 
), decreased energy level, slowed thinking, and a low blood count (anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts... read more ). Low testosterone levels also increase the risk of coronary artery disease Overview of Coronary Artery Disease (CAD) Coronary artery disease is a condition in which the blood supply to the heart muscle is partially or completely blocked. The heart muscle needs a constant supply of oxygen-rich blood. The coronary... read more 
.
Testosterone-Replacement Therapy
Many men with normal testosterone levels are interested in taking testosterone to slow or reverse development of characteristics of low testosterone. However, currently testosterone replacement therapy (TRT) is recommended only for men with both symptoms of low testosterone and abnormally low blood levels of testosterone, a condition called hypogonadism Decreased Libido in Men Decreased libido is a reduction in sex drive. Possible causes include psychologic factors (such as depression, anxiety, or relationship problems), drugs, and low blood levels of testosterone... read more . Recent studies have found conflicting evidence suggesting a possible increased risk of heart attack and stroke in men who take TRT.
Side effects of testosterone-replacement therapy
Testosterone treatment may rarely have certain side effects, such as snoring, increase in symptoms of urinary tract blockage (usually caused by benign prostatic hyperplasia Benign Prostatic Hyperplasia (BPH) Benign prostatic hyperplasia (BPH) is a noncancerous (benign) enlargement of the prostate gland that can make urination difficult. The prostate gland enlarges as men age. Men may have difficulty... read more ), mood changes, acne, blood clots, and breast enlargement Breast enlargement in men Breast disorders occur infrequently in men. Breast disorders include Breast enlargement Breast cancer Breast enlargement in males is called either gynecomastia or pseudogynecomastia. Gynecomastia... read more . Testosterone sometimes causes the body to make too many red blood cells, possibly increasing the risk for various disorders, such as blood clots and strokes.
Currently, testosterone treatment is thought to have no effect on the development or progression of prostate cancer Prostate Cancer Prostate cancer begins in a small area of the prostate gland, an organ found only in males. The risk of prostate cancer increases as men age. Symptoms, such as difficulty urinating, a need to... read more 
. However, this subject is not entirely understood, and men should talk with their doctors about their risk of developing prostate cancer.
Follow-up to testosterone-replacement therapy
Men taking testosterone need to be checked every few months for changes in their blood count and for prostate cancer. Such testing may detect cancers early, when they are more likely to be curable. Some men with prostate cancer may take testosterone treatment, but they should be checked even more frequently by their doctor.
