The Massachusetts Male Aging Study (MMAS) documented an inverse correlation between ED risk and high-density lipoprotein (HDL) cholesterol levels but did not identify any effect from elevated total cholesterol levels.  Another study involving male subjects aged 45-54 years found a correlation with abnormal HDL cholesterol levels but also found a correlation with elevated total cholesterol levels. The MMAS included a preponderance of older men.
Both ED and low testosterone (hypogonadism) increase with age. The incidence of the latter is 40% in men aged 45 years and older.  Testosterone is known to be important in mood, cognition, vitality, bone health, and muscle and fat composition. It also plays a key role in sexual dysfunction (eg, low libido, poor erection quality, ejaculatory or orgasmic dysfunction, reduced spontaneous erections, or reduced sexual activity). 
Conditions associated with reduced nerve and endothelium function (eg, aging, hypertension, smoking, hypercholesterolemia, and diabetes) alter the balance between contraction and relaxation factors (see Pathophysiology). These conditions cause circulatory and structural changes in penile tissues, resulting in arterial insufficiency and defective smooth muscle relaxation. In some patients, sexual dysfunction may be the presenting symptom of these disorders.
I have no idea why your boyfriend's junk is stuck in the junkyard. He's just 25, in the prime of his penis. It's fairly rare for a guy that young to go from instant erection to shy willy. One study found that just 2 percent of men experienced erectile dysfunction before the age of 40. If it's a short-term thing, it could be simple: Maybe he just needs to get more sleep or have fewer drinks.
In the meantime, make sure he gets some rest and takes it easy. The more you two stress about it — and the more pressure he feels you are putting on him — the harder it will get for him to get hard, in all likelihood. So take a few days off. Relax, be patient, and help him find him some help if he needs it. In the meantime, here's a Cosmopolitan.com guide about exactly this topic from sex therapist Dr. Jane Greer.
Medical conditions, such as hypertension, diabetes mellitus, and cardiovascular disease (CVD), and psychological conditions, such as depression and anxiety, also contribute to sexual dysfunction in middle-aged or elderly men. CVD and hypertension cause a narrowing and hardening of the arteries, leading to reduced blood flow to the corporal bodies, which is essential for achieving an erection. Diabetes is a common aetiology of sexual dysfunction, because it can affect both the blood vessels and the nerves that supply the penis. Men with diabetes are four times more likely to experience ED, and on average, experience ED 15 years earlier than men without diabetes.7 Obesity is also correlated to the development of several types of dysfunction, including a decrease in sex drive and an increase in episodes of ED.8
The risk of impotence increases with age. It is four-fold higher in men in their 60s compared with those in their 40s according to a study published in the Journal of Urology (2000;163:460-463). Men with less education are also more likely to experience impotence, perhaps because they tend to have less healthy lifestyles, eat a less healthy diet, drink more and exercise less. Physical exercise tends to lessen the risk of impotence.
Just as certain meds can make it difficult for men to have an orgasm, some can keep the flagpole from even getting raised in the first place. Anti-depressant medications like Prozac and Zoloft, anti-anxiety pills like Valium, high blood pressure medicine like Diuril, and even over-the-counter cold medicines like Sudafed and anti-heartburn pills like Zantac can inhibit erections.
Acetylcholine released by the parasympathetic nerves is thought to act primarily on endothelial cells to release a second nonadrenergic-noncholinergic carrier of the signal that relaxes the trabecular smooth muscle. Nitric oxide released by the endothelial cells, and possibly also of neural origin, is currently thought to be the leading of several candidates as this nonadrenergic-noncholinergic transmitter; but this has not yet been conclusively demonstrated to the exclusion of other potentially important substances (e.g., vasoactive intestinal polypeptide). The relaxing effect of nitric oxide on the trabecular smooth muscle may be mediated through its stimulation of guanylate cyclase and the production of cyclic guanosine monophosphate (cGMP), which would then function as a second messenger in this system.
Most older men suffer not ED but erection dissatisfaction. Starting around age 50 (often earlier among smokers and/or diabetics), erections change. In some men, the process is gradual, in others, it happens more quickly. Either way, older men lose the ability to raise erections solely from sexual fantasies. Direct fondling of the penis becomes necessary. When erections appear, they rise more slowly and do not become as firm as they were during men’s thirties and forties. And minor distractions may cause wilting—the doorbell or an ambulance siren. These changes alarm many men, who jump to the conclusion that they must have ED. If you can still raise erection durings masturbation, you don’t. What you have is erection dissatisfaction.
ED means no erections from masturbation. According to the American Urological Association, ED is “the inability to achieve or maintain an erection sufficient for satisfactory sexual performance.” Huh? That’s absurdly vague. If you define “an erection” as what you see in porn, and “satisfactory sexual performance” as porn sex—instant, hard-as-rock erections that last forever with climaxes always on cue—then just about every guy has ED. What is ED, really? For practical purposes, it means that a man who’s sober (no alcohol or other erection-impairing drugs) cannot raise even a semi-firm erection after extended masturbation.
Not to give your already stressed-out dude one more thing to worry about, but stress is the cause of 20 percent of all erectile problems, from one-off boner blunders to a lingering inability to get and maintain an erection. Of course, sex difficulties are just the tip of the stress-induced health problem iceberg — sustained stress can also lead to insomnia, stomach troubles, chest pains, anxiety, and more severe health issues in the long term.
In their extensive review, Bassil and coworkers summarise the benefits and risks, with benefits such as improvement of sexual function, bone density, muscle strength, cognition and overall improvement in quality of life. Among the risks that have been suggested include erythrocytosis, liver toxicity, worsening of sleep apnoea and cardiac function, possibly increasing symptoms of benign prostatic hyperplasia (BPH). They also note that although a possibility of stimulation of prostate cancer has been hypothesised, no scientific or clinical evidence exists to this possible risk.38
Physicians make a diagnosis of erectile dysfunction in men who complain of troubles having a hard enough erection or a hard erection that does not last long enough. It is important as you talk with your doctor that you be candid in terms of when your troubles started, how bothersome your erectile dysfunction is, how severe it is, and discuss all your medical conditions along with all prescribed and nonprescribed medications that you are taking. Your doctor will ask several questions to determine if your symptoms are suggestive of erectile dysfunction and to assess its severity and possible causes. Your doctor will try to get information to answer the following questions:
Diet can also affect a man’s ability to achieve or maintain an erection. As stated, men who indulge in alcohol may have trouble with erections, but men who have poor diets, suffer from diabetes, or who are overweight can also restrict blood flow to the penis or suffer from poor body image. All of these factors, especially when combined, can lead to erectile dysfunction.
Alteration of NO levels is the focus of several approaches to the treatment of ED. Inhibitors of phosphodiesterase, which primarily hydrolyze cGMP type 5, provided the basis for the development of the PDE5 inhibitors. Chen et al administered oral L-arginine and reported subjective improvement in 50 men with ED.  These supplements are readily available commercially. Reported adverse effects include nausea, diarrhea, headache, flushing, numbness, and hypotension.
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.
Men, if you can't get an erection and are in a relationship with someone you deeply care about... Please.. Bring on the toys.. bring on the hands.... bring on the tounge.. do SOMETHING... Don't use that as an excuse not to erouse the woman you love. If shes not getting satisfied from you, she will find it somewhere else OR... she will be sad stuck in a relationship STARVING for sex and have pity on you. You can do so much without an erection. That's not the end all be all in orgasms for women. Trust me... You need to over compensate for problem. You can actually appear MORE manly by stepping up and making sure woman is satisfied. Don't let your bed be a graveyard.!!! I am pretty sure you can still have an orgasm without an erection.. if you have the right woman,... pleasure her. Do what it takes to help her orgasm... and then it's your turn. She will make sure you are pleased regardless if you are fully erected or squishy... Don't fret over this.. Seriously...!!! Don't feel like your manhood is any less... Rise up and serve and she will make you happy. Trust me!!
Remember what I said before about how it's not you? Okay, sometimes it is you. But it's not that you're not sexy — it's that for men, as well as women, relationship problems (like fighting all the time, or having clashing expectations about where things are going) can severely mess up your sex drive and ability to become aroused. Which makes sense — if you're spending 90 percent of your time together fighting about whether you're going to move in, switching gears to make 10 percent of your time together into a sexy sex party is pretty damned difficult.
Lifestyle choices that impair blood circulation can contribute to ED. Smoking, excessive drinking, and drug abuse may damage the blood vessels and reduce blood flow to the penis. Smoking makes men with atherosclerosis particularly vulnerable to ED. Being overweight and getting too little exercise also contribute to ED. Studies indicate that men who exercise regularly have a lower risk of ED.