Is there something that starts in teenagers that causes them to later get coronary atherosclerosis?

Yes: The most famous example is from autopsies of us soldiers (ages 18 and above) who died in vietnam. The aortas of many demonstrated fat-laden cells which are the first step toward atherosclerosis. No data, to my knowledge about corresponding lifestyles, but we know many were smokers.

Yes, Many...: 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. Lipoprotein concentrations are the primary driver. Arteries enlarge, not shrink. Though ivus commonly detects in teenagers, plaque rupture & clots produce narrowing, typically decades later.