Hypercholesterolemia running in a family would start at what age in the kids?

Very early. Changes in blood vessles is a life long event and familial cholesterolemia can make this worse right from the start. Everyone in your family should know their cholesterol and be very aware of healthy eating habits.
Before age 10 in FH . Familial hypercholesterolemia (fh) is a genetic disorder causing decreased clearance of LDL from blood. Current nih recommendations call for all children age 9-11 to have lipid screening.