I generally *always* call people who are older than me by Miss/Ms./Mr./Mrs./Dr. or whatever. I have a hard time calling *anyone* who is older than I am by their first names. I am not sure why, though!
When I was a child, my neighbors across the street called my parents Miss Mary and Mr. Dave, but it kind of bothered me, and I could never call their parents anything other than Mr. and Mrs. Dukowski! It also really irritates me that a lot of my fellow grad students call professors by their first names -- I feel like they shouldn't be allowed to do that until they also have a Ph.D. after their names!
