What causes ED in men in their twenties? Can it be reversed?

Stress, etc., &often. The most common cause in someone without serious health issues is psychologic. Stress is a huge factor, as is the nature of your intimate relationships. However some other causes can be from medications such as many blood pressure meds, diabetes, alcohol, and circulation problems. Low testosterone is less likely in a young person. Find the cause and improvement is likely!
Anxiety. The most common cause of ED in relatively young men is performance anxiety. Alcohol over use is also a common cause as well. An understanding that this is a common occurance, a stable and supportive partner, and developing confidence typically resolve this issue.