dear, there are many cause to ed. At your age atherosclerosis can be a major issue leading to ED. as the age progresses, our body mechanisms don t stay that efficient to sustain an erection that long. In individuals that have heart issues or diabetes problems, it further adds to the problem. in your case before commiting to any diagnoses i would need to know... do you feel the desie to have sex? do you feel tired or get exhausted during intercourse? do you get penile erection during sleep? I advice you a complete lipid profile and ecg and blood sugar level. Stay away from viagra as it may lead to cardiac complications in your case. thanks regards dr pankaj