What are the chances of developing an arrhythmia or a heart condition at age 21 without any history of heart issues, or other health issues?

Assuming absence of congenital heart malformations, a lightening strike, or a vaccine causing myocarditis, the chances of a heart attack or life threatening cardiac arrhythmia, would be almost nil in an otherwise healthy 21 yo.

I might add to Dr. M's advice that,providing you don't have a family history of fainting,sudden deaths,known familial EKG abnormalities such as long QT syndrome and your EKG is normal, your chances of having a primary cardiac arrhythmia is exceedingly.low.