How common is small fiber neuropathy for someone who is 31 and no known risk factors?

Not fully rare: Since there are multiple causes, this is not that unusual. The most common is diabetes and pre-diabetes, but can be seen with hereditary disorders such as Ehlers-Danlos; also lupus, gluten intolerance, Sjogren's, sarcoidosis, and variety of rarer problems. Often diagnosis is overlooked or minimized due to presentation of pure sensory involvement causing decreased sensation to pin and temperature.