I think it's definitely a class thing. I'm a northeasterner too, and I never called any adult by their first name. Always Mr./Mrs. Last Name, and real aunts and uncles only were called Aunt/Name First Name. Close family friends were Mr./Mrs. Last Name.

Christian grew up poor in Dayton. They were/are much less formal and it made me super uncomfortable at first when his parents wanted me to just call them by their first names.