Sexual activity is an instinctive, automatic part of human behavior. But the biology behind that behavior is quite complex, particularly in men. For many men, sexuality is one of the things that change over time. It's usually a gradual, almost unnoticeable process that begins in middle age.
How Age Affects Sexual Function
Scientists don't understand all the factors that contribute to sexual function in young men, let alone what is responsible for the changes that occur with healthy aging. Still, it's clear that the hormones, nerves and blood vessels responsible for male sexuality all change over time.
Although the sex act is a continuous process, it has six stages. Almost all are affected by aging.
1. Sexual Desire or Libido
The normal sex drive is a prime example of the unity of mind and body: It requires both the right mind set and enough testosterone, the male hormone. Sexual desires surface in puberty, when testosterone levels rise. Although the intensity of the sex drive tends to wane with age, most men make enough testosterone to maintain libido throughout life.
For most older men sexual interest is still there, but it's generally a far cry from the preoccupation with sex that's so common in youth. Many older men think about sex but don't have the drive to put theory into practice. And even when the spirit is willing, the flesh may be weak.
On average, levels of testosterone fall by about 1% per year beyond age 40, but most older men still have enough testosterone to function sexually. To produce arousal, testosterone acts on the brain, and these nerve cells become less hormone-responsive with age. In contrast, men tend to produce more female hormones (estradiol and prolactin) as they age.
Sexual activity begins with arousal. It's the result of a combination of erotic thoughts and sensory stimulation that may involve touch, sight, scent, taste or hearing. An area of the brain called the hypothalamus coordinates erotic images and sensations and transmits the impulses of desire through the spinal cord to the pelvis. The impulses link up with the nerves in the pelvis, which spring into action. Sensory nerves from the skin of the penis and other erogenous zones also connect directly to the pelvic nerves, bypassing the brain. All of this happens involuntarily.
When properly stimulated, the pelvic nerves send chemical signals to the arteries of the penis. This causes them to widen and admit more blood. Blood rushes in and the penis swells. This causes an erection. For years, doctors have known that an erection is a hydraulic event (it requires pressure created by forcing liquid through a narrow "pipe") that depends on a six-fold increase in the amount of blood in the penis. But new research has revealed that an erection is also a chemical event. A tiny chemical called nitric oxide allows nerves to communicate with each other and with the arteries of the penis. It's been an exciting discovery and has led to the development of erectile dysfunction (ED) medications (Viagra, Levitra, and Cialis).
As for nerve function, penile responsiveness to sensory stimulation slows with age. In addition, penile blood flow may decrease as men grow older, even if they stay healthy.
Most men experience decreased sexual responsiveness with increasing age. Erections occur more slowly and they become more dependent on physical stimulation than on erotic thoughts.
Even when erections develop, most men in their 60s report that they are more difficult to sustain and not as hard or rigid.
Nighttime erections diminish with age. Between the ages of 45 and 54, men average 3.3 erections per night; between ages 65 and 75, it's 2.3 erections. The erections also tend to become briefer and less rigid with age.
This stage usually lasts from 30 seconds to two minutes. Heart rate and blood pressure increase and more blood is pumped to the body's tissues. Blood flow increases not just to the penis; most men also experience facial flushing and the testicles swell by about 50%. During the plateau phase, the prostate and seminal vessels begin to discharge fluid to prepare for ejaculation. There are no reported changes in the plateau phase with aging.
During this fourth stage, the muscles in the epididymis, vas deferens, seminal vesicles and prostate contract automatically. This propels semen forward. At the same time, nerve impulses tighten muscles in the neck of the bladder so that semen is forced out through the urethra instead of flowing back into the bladder. The pleasurable sensation of orgasm usually occurs with ejaculation. In nearly all men, the heart rate reaches its peak during ejaculation.
Ejaculation also changes with age. The muscular contractions of orgasm are less intense, ejaculation is slower and less urgent, and semen volume declines.
Sperm counts also decline. Healthy men can father children well into their senior years, but their fertility rate can't match younger men's.
All good things come to an end. Detumescence is when the penis returns to its flaccid state. It usually follows ejaculation, but it can occur prematurely if the sex act is interrupted by an intrusive thought or event. In either case, detumescence occurs when the penile arteries narrow and the veins widen, draining blood away. There are no reported changes in this phase due to aging.
6. Refractory Period
The final stage in the sex act is the quietest. It lasts from 30 minutes (in younger men) to 3 hours (in older men). The penis can't respond to sexual stimulation during this phase.
At any age, however, worry, stress or depression can interfere with sexual interest, activity and satisfaction even if a man's physical apparatus remains intact. So, too, can marital strife, poor communication, poor sexual technique and boredom; many of these problems become more common with age.