What are the chances that i could have a heart attack at the age of 21?

Low: Low unless using cocaine, glue, snuffing hydrocarbons, have critical aortic stenosis, severe pulmorary hypertension, idiopathic cardiomyopathy, accelerated arteriosclerosis, uncontrolled diabetes, hyperlipidemia, or coronary spasm, congenital heart deisease to include some.

Near zero: Unless you have an autoimmune disease like lupus, take Cocaine or methamphetamines, or have terrible genetic predisposition to atherosclerosis, it is nigh on impossible to have a heart attack @ 21. Since you've survived to 29, hopefully someone has worked out why or how you did have one 8 years ago, or it was just a panic attack(which is a serious disease in its own right, but is less often fatal.