How probable is it for a 21 year old to have a heart attack?

Unlikely: The most common way the heart muscle becomes damaged is when blood flow to the heart muscle is impaired. The most common mechanism is atherosclerosis which would be exceeding unlikely in a 21 year. If a 21 sustained damage to the heart muscle it would be more likely from toxins such as alcohol, methamphetamine and cocaine. Some congenital abnormalities cause heart rhythm problems.

Unusual: Possible congenital coronary anomalies are known and can cause ischemia. Aortic dissections can occur and involve coronaries. Even atherosclerosis can occur in some high risk hyperlipemia patients. Drug abuse- Cocaine has been identified.