What are the symptoms of erectile dysfunction in a young man?

Erectile dysfunction: Difficulty getting and/or maintaining an erection.

Same as in older man: By definition, erectile dysfunction is the inability to attain and/or maintain an erection adequate for sexual activity to such a degree that it causes distress. Age is irrelevant in the definition but distress is a requirement. If the lack of an adequate erection does not cause stress, then you don't have erectile dysfunction, regardless of age.