Sinus bradycardia is completely normal in young healthy people. So is sinus arrhythmia. They usually occur together as a result of high vagal nerve activity.
I wouldn't worry about the interpretation saying old septal infarct. The chance of a 18 y/o male having had a heart attack without noticing it is so slim that it's not worth even thinking about. Especially since the cause usually is electrode misplacement or more common, the way the heart is positioned in the chest (which can vary a bit from person to person).
Normally he would have. He must have concluded that it is unnecessary.
However, because of the results they came up with, I would consult a cardiologist. First, to know what the consequences are of the sinus arrhythmia. Second, if you can exercise without limitations. Third, if you need to take lifelong statins to avoid future problems from your high cholesterol.
Do not be too alarmed by the low heart rate. I do not know what normally your resting pulse is, but in itself, a heart rate of of below 50 is not necessarily dangerous. Many athletes have low resting heart rates while being in perfect health condition.