I'm lowering the dose of clonazepam from 1 mg to 0, 3 now. My question is: what harm can this med do to a 24 year person in long term taking?

Do this with a psych. Do begin by discussing this move with the doctor who prescribed it . I am concerned that there is more here than meets the eye initially. That's doctor speak for get the excellent psychiatric attention you deserve. As they say, don't try this at home. Call tomorrow. Ok.
Addiction. I am not sure about your dosing. Regardless, a good rule to follow is that any long term medicine is to improve your quality of life. Not blunt the moment. When your long term lifestyle or family members are adversely affected, there is usually a problem. A good rule for anxiety medicine is to never take one on a schedule but when you absolutely need.