Circulation problems was actually my first thought as well. Age is not the only thing that affects circulation. In fact, diabetes is a much bigger culprit than age. Also smoking and several other things can adversely affect circulation. And that will cause hair loss in the lower legs that creeps up.
However, socks pulling hairs out can also cause this. Mainly if they are tight you pull them out when pulling socks on or off. Pulling out hairs can damage the hair follicle such that hair doesn't grow back (eyebrows is where you see this most often).
Also, weight lifting can cause this if you use that part of your leg... it pulls the hairs out where equipment rubs. This normally would not be all the way around the leg though and would be specific spots where the leg contacts the equipment.
The cure depends on which problem it is.
Socks & circulation... If you're wearing socks that are too tight for your leg/ankle for hours at a time, you can reduce circulation enough to maybe produce a problem. When you take your socks off, is there a noticeable indentation in your leg/ankle? That could be a sign. Also, do your legs swell above your socks (like fluid is being held above your sock line)?
Generally speaking, with diabetes there would likely be other signs and symptoms of poor circulation if that were the case. Do your feet (especially toes) feel cool or cold to the touch compared with the rest of your leg? Do you experience numbness or tingling in your feet?
Also, if you're a man 18-39 who's in good shape (not overweight), it's unlikely you have diabetes, even with a family history. There are two general types of diabetes, Type 1 and Type 2.
Type 1 typically occurs during childhood. If you had that,
you would very likely know it. These patients often find out in the emergency room because their blood sugar has gotten so dangerously high that they literally get sick. Or they have extremely frequent urination, excessive thirst and hunger bad enough to make them seek medical attention.
Type 2 typically occurs in people who are significantly overweight and usually later in life 40+. Although there is now what is being called Type 1 & 1/2 diabetes which occurs in overweight, sedentary young people (12-25-ish).
Certain skin conditions can absolutely cause alopecia (hair loss). Very dry, scaly skin that keratinizes excessively (gets thick and tough) will literally block hair from growing out of the follicle. Skin infections, like some fungal infections, can affect follicle health.
Bottom line: there are too many possibilities that any of us could conjecture about. The best way to get the right answer is to see a doctor to have any or all of these possibilities eliminated.
My advice... start with a dermatologist. They are specialists in hair as well as skin. A good one will be less likely to guess repeatedly before finding the answer.
