Pitting happens with age. Can be due to grit getting into the grease, subtle ovalization of the bearings, work hardening of the cone surface or all of the above.
If I were you, I'd find the oldest bike shop in the area and take it in there to see if they have a replacement. The size and thread of yours is common, so it shouldn't be too big a deal. Get some fresh bearings too while you're at it.