I would think the 25% is just an average. Some men never have a problem with erections. Some healthy men in their 60's never have a problem with their erections -- no Viagra needed at all, thank you. And, no, it is NOT completely associated with age. In fact, many men in their 40's and even younger, are already dealing with ED, especially if they smoke, are obese, don't exercise, or are addicted to porn, among other things. In fact, even men in their 20's and 30's can have these problems.
Much of the emphasis on erectile pathophysiology has been placed on penile smooth muscle function and cavernosal hemodynamics. The neuroanatomy and neurophysiology of erection can be characterized but its full extent is poorly understood. Neurologic disease does not always reproducibly affect erections in a uniform manner compared to other types of sexual dysfunction (SD). This offers many obstacles to understanding the role the nervous systems plays in SD and consequently obscures what treatment options readily optimize erections specific to the neurologic insult.
Alprostadil is a potent vasodilator and smooth muscle relaxant identical to the naturally occurring PGE1. PGE1 binds with specific receptors on smooth muscle cells and activates intracellular adenylate cyclase to produce cAMP, which in turn induces tissue relaxation through a second messenger system (96). PGE1 is the only FDA approved form of intracavernosal therapy and is available commercailly as EDEX, or Caverject. Its efficacy was demonstrated in several clinical trials where the rate of responders ranged from 40% to 80% (97,98). The most common adverse event is penile pain, which is not related to the injection of the medication itself. In men with prolonged use the pain is usually self-limited (99).
There have been rare reports of priapism (prolonged and painful erections lasting more than six hours) with the use of PDE5 inhibitors such as sildenafil, vardenafil, and tadalafil. Patients with blood cell diseases such as sickle cell anemia, leukemia, and multiple myeloma have higher than normal risks of developing priapism. Untreated priapism can cause injury to the penis and lead to permanent impotence. Therefore, if your erection lasts four hours, you should seek emergency medical care.
Impotence is a common problem among men and is characterized by the consistent inability to sustain an erection sufficient for sexual intercourse or the inability to achieve ejaculation, or both. Erectile dysfunction can vary. It can involve a total inability to achieve an erection or ejaculation, an inconsistent ability to do so, or a tendency to sustain only very brief erections.
Depression and anxiety: Psychological factors may be responsible for erectile dysfunction. These factors include stress, anxiety, guilt, depression, widower syndrome, low self-esteem, posttraumatic stress disorder, and fear of sexual failure (performance anxiety). It is also worth noting that many medications used for treatment of depression and other psychiatric disorders may cause erectile dysfunction or ejaculatory problems.
Stress is your body responding to your environment. And it’s a good thing—in limited doses. When you get stressed out your body makes chemicals like adrenaline that make you stronger, faster, fitter, and even able to think more clearly. Most people call this reaction the “fight-or-flight” response, and it’s a life-saver in dangerous situations. In a very real sense, adrenaline makes you a part-time superhero. The problems happen when your body deals with constant stress.
It is estimated that up to 20 million American men frequently suffer from impotence and that it strikes up to half of all men between the ages of 40 and 70. Doctors used to think that most cases of impotence were psychological in origin, but they now recognize that, at least in older men, physical causes may play a primary role in 60% or more of all cases. In men over the age of 60, the leading cause is atherosclerosis, or narrowing of the arteries, which can restrict the flow of blood to the penis. Injury or disease of the connective tissue, such as Peyronie's disease, may prevent the corpora cavernosa from completely expanding. Damage to the nerves of the penis, from certain types of surgery or neurological conditions, such as Parkinson's disease or multiple sclerosis, may also cause impotence. Men with diabetes are especially at risk for impotence because of their high risk of both atherosclerosis and a nerve disease called diabetic neuropathy.
Malleable implants usually consist of paired rods, inserted surgically into each of the corpora cavernosa. The rods are stiff, and to have an erection, one bends them up and then when finished with intercourse one bends them down. They do not change in length or width. The malleable implants are the least mechanical and thus have the lowest risk of malfunction. However, also have the least "normal appearance."
Injection therapy: The modern age of such drug therapies began in 1993 when the injection of papaverine (Pavabid), an alpha-blocker that produces vasodilatation (widening of the blood vessels), was shown to produce erections when injected directly into the penis. Soon afterward, other vasodilators, such as prostaglandin E1 (PGE 1) monotherapy (Caverject, Edex), PGE1 and phentolamine (Regitine), and Trimix (papaverine, phentolamine and prostaglandin E1), were demonstrated to be effective. The benefit of combination therapy is the decreased dosing of each with less side effects. Most important is the reduction of the prostaglandin PGE1 dosing, which is associated to the localized pain.
Oral therapies via the PDE5i sildenafil, vardenafil, and tadalafil have been proven to be generally safe and effective in select NED populations. The majority of the treatment effectiveness data has been generated in the SCI population. Data regarding the use of PDE5i outside of the SCI population is lacking (58). Furthermore, the ED that exists in the population with neurologic disorders is often multifactorial and may be caused by psychogenic, psychosocial, hormonal, medication-related and disability-related factors. A careful evaluation of each patient must be performed to isolate these factors prior to initiating vasoactive therapy.
In this study, ED proceeded CVD in almost 70% of cases. Similarly, many men with ED have been found to have pre-existing CVD. A study by Vlachopoulos et al evaluated the incidence of asymptomatic CVD in 50 men with ED.22 These authors found that 19% of men with ED had asymptomatic CVD. Similarly, Mulhall and colleagues found that 20% of men presenting with ED and vascular insufficiency on penile duplex had asymptomatic CVD.23
Risks associated with injection therapy including bleeding, pain with injection, penile pain, priapism, and corporal fibrosis (scarring inside of the corpora cavernosa). There is also concern that repetitive injections in the same area could cause scar tissue to build up in the tunica albuginea that could create penile curvature. Thus, doctors recommended that one alternate sides with injection and perform injections no more frequent than every other day.
One of the first steps is to distinguish between physiological and psychological ED. Determining whether involuntary erections are present is important in eliminating the possibility of psychogenic causes for ED. Obtaining full erections occasionally, such as nocturnal penile tumescence when asleep (that is, when the mind and psychological issues, if any, are less present), tends to suggest that the physical structures are functionally working. Similarly, performance with manual stimulation, as well as any performance anxiety or acute situational ED, may indicate a psychogenic component to ED.
In a prospective, multicenter, single-armed study of ED patients who exhibited a suboptimal response to PDE5 inhibitors, the investigators found that percutaneous implantation of zotarolimus-eluting stents in focal atherosclerotic lesions was both safe and feasible and was associated with clinically meaningful improvement on subjective and objective measures of erectile function. 
Their treatment plan will include a great deal of information about ED. It is important you take the time to read it all. You will be better prepared to manage your condition as a partner—and not just a patient. Also, erectile dysfunction is often a symptom of a more serious underlying condition—like heart disease, diabetes, high blood pressure, or even depression. That’s why we want you to learn as much as you can. Nothing would be a sign of our success more so then if you could resolve the condition that causes your ED instead of needing to use the medications your doctor prescribes. We strongly recommend optional laboratory tests. You do not need to get them to receive treatment but it can be one of the best things you can do for your health in the long run.
Following a detailed discussion about the history of erectile dysfunction and its risk factors, your doctor will examine the testicles and penis to help determine the cause of erectile dysfunction. Your doctor will check reflexes and pulses in the area to see if problems with blood vessels or nerves are contributing to the erectile dysfunction. If necessary, your doctor will order tests to help diagnose erectile dysfunction.
You should talk to your doctor about possible treatments. You may want to talk to other patients who have had the treatment planned for you. You also may want to seek a second doctor's opinion about surgery before making your decision. You may find it difficult to talk to your doctor about impotence. You will want to find a doctor who treats this condition and will help you feel comfortable talking about the problem and choosing the best treatment. You can also get more information by contacting your local National Kidney Foundation affiliate.
Other medical therapies under evaluation include ROCK inhibitors and soluble guanyl cyclase activators. Melanocortin receptor agonists are a new set of medications being developed in the field of erectile dysfunction. Their action is on the nervous system rather than the vascular system. PT-141 is a nasal preparation that appears to be effective alone or in combination with PDE5 inhibitors. The main side effects include flushing and nausea. These drugs are currently not approved for commercial use.
ED varies in men with seizure disorders, occurring in 3% to 58% of men with epilepsy (30). The cause of ED is likely multifactorial, with neurologic, endocrine, iatrogenic, psychiatric and psychosocial factors leading to varying degrees of ED (31). ED can occur in periods surrounding active seizures (ictal) or in the periods unrelated to seizure activity (post-ictal) as well (32).
The book is really full of Very Important sexual related Advise to the men affected by Erectile Dysfunction, ED. The Author really took time and researched fully about the condiion, It has some of the likely causes of the condition so you can use it as a manual to check on yourself , conditions contributind to the ED conditions include Heart disease, Diabetes, Pornography and Masturbation, smoking and substance abuse among others. Likely complications and Treatments for the conditions are well laid i this well written book.
Centrally active compounds such as apomorphine have been used in men with ED whose cardiovascular comorbidity may prohibit PDE5i use, or in men who have concurrent apomorphine use for its anti-parkinsonian properties. Unfortunately, its side effect profile and poor effectiveness compared to other ED treatments have impaired its mainstream utilization (118). It is suspected that the side effects of apomorphine relate to its D2 receptor affinity. D4 receptor agonists, such as ABT-724 and azulenylmethylpiperazines, may not have the same associated side effects and show potent pro-erectile effects in animal models compared to apomorphine (32,119).
The link between chronic disease and ED is most striking for diabetes. Men who have diabetes are two to three times more likely to have erectile dysfunction than men who do not have diabetes. Among men with erectile dysfunction, those with diabetes may experience the problem as much as 10 to 15 years earlier than men without diabetes. Yet evidence shows that good blood sugar control can minimize this risk. Other conditions that may cause ED include cardiovascular disease, atherosclerosis (hardening of the arteries), kidney disease, and multiple sclerosis. These illnesses can impair blood flow or nerve impulses throughout the body.