Is there many cases where kids/teenagers get coronary atherosclerosis?

No: Adolescents/children are more likely to have congenital heart malformations such as a patent foremen ovale (hole between the right and left side of the heart) rather than symptomatic coronary atherosclerosis. However, early athersclerotic changes have been observed in arteries of some children. This is especially true for children at risk with obesity, hypertension, or elevated cholesterol levels.

Rare: There are rare genetic anomalies called progeria (see http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0002622/) and werner's syndrome (no relation to me). (see http://ghr.Nlm.Nih.Gov/condition/werner-syndrome) but both of these are very rare. Classic studies in the 50s showed 19 year olds killed in korea had early signs of arteriosclerosis - so we know that the disease begins early in life.

Onset Age 7: Atherosclerosis has been known, for over 7 decades, from autopsy data, to typically begin in later childhood, age 7 y/o typical, earlier with obesity and/or diabetes, yet remains asymptomatic for decades. Arteries enlarge, not shrink. Though ivus commonly detects in teenagers, plaque rupture & clots produce narrowing, typically decades later. Thus symptomatic atherosclerosis in teenagers is rare.