I've had difficulty for a few years now and just turned 50 this year. The Dr's just prescribed pills and had no ideas about root cause. I found the dosage I needed was increasing so I really focused on making changes a couple months ago. I made so many it's hard to tell what has worked but I've listed them in the order that I think matters most. I've had morning wood every day for the past month+ and hadn't hardly at all for the prior couple years. Things are much better right now.
From this site and others there seem to be two possible causes: circulation problems or hormone imbalance. In either case I think of ED as like the canary in the coal mine - an early indicator of poor health. This was a surprise to me because I exercise regularly, am not overweight, and my diet was probably better than the average American (but that is not saying much). In any case, when in doubt my focus has been on strategies to improve my overall health.
1. Diet. I never ate a lot of meat/dairy but am not a vegetarian either. Still I wasn't eating even the minimum recommendation of fruits and vegetables. About a month and a half ago I started drinking about a quart of green smoothie every day. About 1/3 to 1/2 fruit (berries, apple, banana, peaches - whatever), a little juice, and the rest greens (kale, romaine, spinach, chard, broccoli - whatever). If you google there are some good books and websites on the subject but I don't follow any recipes, I just throw in what I like or have on hand. Things I don't like by them-self are generally still good in a smoothie. Then I add a little protein powder and flax seed. Not quite as good as a fruit smoothie but much better tasting than it sounds (or looks). This is the simplest possible way I can imagine to make such a dramatic change in diet. I drink 1/2 in the morning and the other half after a work out or whenever. I get about 8 servings of fruits and vegetables just from the smoothie. So I went from almost never getting the minimum recommendation of fruits and vegetables to always getting the minimum and sometimes much more. All it takes is a good blender and about 10 minutes. I had a pretty strong blender before but recently upgraded to a Blendtec which does a better job and will blend anything.
2. Reduce soy and beer. I used to drink soy milk and soy protein. There is some evidence that soy can mimic estrogen. While the Asians consume a lot of soy w/o ED problems, they get it in a very different form. No more soy milk or soy protein. For a time I switched to whey protein but am trying to minimize animal products so I'm now taking pea, rice, and hemp protein. There is a great site: True Protein, just renamed to True Nutrition where you can create custom protein mixes at better prices after shipping than you are likely to find anywhere. Beer contains hops which is very estrogen promoting, so I'm down from 1 to 2 beers per day to only drinking it occasionally. I sometimes drink red wine instead now.
3. Red rice yeast. My cholesterol was not high but my ldl and hdl were each at the wrong end of the range. Red rice yeast is probably the best supplement for improving cholesterol. My Father-in law had heart disease for years and was having severe side effects from the statins. The Dr's couldn't help him so my wife found this for him a few years ago. It worked much better than the statins with no side effects. I use Bioactive Nutrients Cholesterol Control but red rice yeast is the primary ingredient and widely available.
4. Multi-vitamin (Nature made multi for him), and fish oil (on the days I don't eat fish), and vitamin D. Most Americans are deficient in vitamin D and some studies have shown a link between vitamin D levels and testosterone levels.
5. Nuts. Instead of snacking on processed food lately I mostly snack on raw pecans, almonds, walnuts.
5. Supplements. Still trying to figure out what makes the most difference. Korean Ginseng, L-Arginine & Pycnogenol, DHEA, Tribulus Terrestris, Horny Goat Weed w/ Maca, ZMA - zinc, magnesium, B-6. I took all these regularly and lately have started cutting back, focusing more on supplements for overall health. Have not yet decided which of these I may continue with.
6. Exercise. I'm only including this last because I have always exercised 2 to 3 times per week: weights, bicycling, or swimming, but am now trying for at least 5 times per week counting walking at least 30 minutes.