Is it possible for a teenager to have blocked arteries?

Very rare. The vast majority of teenagers will not have blocked arteries. Rarely, heavy smokers develop buerger's disease, or thromboangiitis obliterans, a rare disease in which blood vessels swell and become blocked with blood clots. It usually affects men and women ages 20 to 40, but it is possible in teenagers! don't smoke!
Yes. Recent studies have demonstrated plaque in coronary arteries as young as 5 years old. Thus good diet, exercise, and heart heath really start from birth.