A narrowed blood vessel can be a sign of systemic high blood pressure, atherosclerotic cardiovascular disease (whether mild or severe) or the after effects of a vascular occlusion.
Given that you have not had a vascular occlusion, are not a diabetic, this leaves either some level of systemic high blood pressure or just the long-term effects of atherosclerosis in the blood vessels. If you have had repeated physicals with no sign of systemic high blood pressure, then this is likely just the result of a collection of atherosclerosis (cholesterol plaque accumulation inside blood vessels). Some people who have advanced formation of this relative to their age, have high cholesterol; others, not.
I would just make sure that systemic high blood pressure and/or high cholesterol are not things you need to have treated. Other than that, living at a healthy weight, eating a healthy diet, exercising, and not smoking are the things to do to mitigate any risk factors that may cause this narrowing of the blood vessels to turn into a vascular occlusion.
Does that make sense and does that address your concerns?
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My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must see an eye doctor