You know, actually, people in my grandmother's generation (like say, 75+) I will not call by their first name, unles they very, VERY specifically tell me not to call them by Mr./Mrs/Ms Lastname. As informal as I am with myself, I am very concerned with being respectful of older people on this topic.
I'm trying to think if my kids know anyone my parents' age they talk to directly (so the same generational gap as the above)....yes, one set of my parents' friends they have been specifically told they can call by their first names. They don't really talk to many other adults my parents' age or older unless it's a teacher or activity leader, and they use Title. Lastname for them.