If no heart defects and no illegal drug use. what is the youngest one can have a heart attack?

Depends: There is no youngest age, but typically the risks of having a heart attack have to do with risk factors. These factors include high blood pressure, high cholesterol, smoking, diabetes, and family history and a few other less common ones. The more risk factors one has, the higher the risk of heart attack. A person with all of these factors could certainly have a heart attack at a very young age.

Any age: By the term heart attack, i assume you mean coronary artery problems. There is no age limit. Infants born with abnormal coronary arteries can have heart attacks, also children who get heart infections. Family history is also important as some have inherted problems with cholesterol that can cause coronary artery problems.