I am from the South and have noticed this "custom" among staff at doctors' offices. Your post reminds me of something I have been intending to do: notify the front office and have them understand that I am 70 years old and do not appreciate being addressed by the familiar "Joe"; this person should give me a little respect and address me as "Mr. ----."
On a side note, when I was a youngster it never occurred to me to address any adult by his or her first name. And that included all relatives. It just wasn't done.
Forgot to note that you could address a relative by his or her first name but only by putting “Aunt” or “Uncle” in front of it.