Fair question. Being from the South and having been raised under a household rule that says all children address all adults by last name, never by first, I am socially conditioned to think "first name equals disrespect" in my feelings, even though I know better in my head. My father didn't allow us to call any adult by first name, even if that adult specifically invited us to. If the neighbor said, "Call me First Name," my siblings and I were under strict orders to smile, say "Yes, sir," and continue to address him as Mr. Last Name. When I asked how it's showing respect if I'm going against what the neighbor wants to be called, my father's answer was, "I can't help it if other people won't play by the rules." To him, it was just never, ever polite for a child (defined as someone under 18) to address an adult (defined as someone over 18) by first name. With my boyfriend's mother who wanted me to call her by her first name, however, we did reach a compromise with my father where I could call her Mrs. First Name. My father was not born a Southerner, or I suspect it would have occurred to him sooner. As has been mentioned in this thread, Mr. or Miss/Mrs. First Name is quite common in the South.

I do have to remind myself nowadays, that a teenager who addresses me by first name with a smile is not being disrespectful. I was raised in a different generation and a different part of the country. I'm told that if I were still living in the South, I would undoubtedly be called Mrs. and ma'am automatically. It's just that customs are different here. However, when I bristled on another website at having a teenager call me Sweetie, and this met with a 15-year-old girl telling me disdainfully (her exact typing style here) "u need to get wit the times, SWEETIE!" I do think that was blatantly disrespectful.