Typically, would smoking on and off for 1 year 5 per day on and off cause coronary artery disease at 18?

Unlikely: But stop now! it is never easier than this moment to quit. Addiction is severe, and the diseases are horrible!

No: If there is underlying coronary artery disease then smoking could certainly exacerbate this and can cause arterial spasm that might cause myocardial ischemia. But light smoking for 1 year would not cause coronary artery disease. Any 18 year old was diagnosed with a heart attack or angina should have evaluation for congenital heart and arterial disease, hyperlipidemia and abuse of stimulant drugs.