Erectile dysfunction can develop in young adults due to a variety of causes. Psychological stress, performance anxiety, depression, neurogenic disorders peyronie's disease, body image issues, medications, alcohol, smoking, drugs/medications, hormonal imbalance, radiation therapy, kidney failure, metabolic disorders, multiple sclerosis, hypertension, diabetes, prostatitis, urethritis etc. just to name a few. Anxiety and stress play an important role in the development of erectile dysfunction; and is the most common possible cause. At a primary level, I would suggest being a bit relaxed and working on the psychological factors with your partner. You could also see a psychotherapist/ behavioral therapist for the same. It might also be beneficial to get rid of factors like smoking/ alcohol/ drugs, if involved. In case this doesn’t work, I would suggest seeing your primary care physician for an evaluation of the above mentioned causes and appropriate specific treatment.
Hope this helps.
Research the side effects of propecia. They include erectile dysfunction. And in some men, the side effects continue even after you've stopped taking it.