Young. It actually develops at a young age. We learned that when autopsies were done on young persons dying in the korean war long ago. There was evidence already for arteriosclerosis. Of course, the older you get the more likely are you to have it and the more severe it might be. So if you have risk factors or a family history of heart problems, better to be checked out early before the damage is done.
Arteriosclerosis. Fatty streaks the first signs have been found in teenagers on autopsies.