When could atherosclerosis begin?

Even in Pre-teens. While atherosclerosis is detectable in preteens it's consequences typically do the manifest until the 50's or later. Atherosclerosis is primarily a consequence of an unhealthy diet with obesity being the main risk factor.
Age 7 & less Typical. From autopsy studies, 1930s on, atherosclerosis, an accumulation of white blood cells, mostly macrophages, have been observed to have invaded into the walls of arteries (not veins) by about age 7 on average. Rarely, about 1 in 6-8, 000 live births, it is already present at birth. On the other hand, a few individual remain free of this behavior well into adulthood.