This may be an old thread, but it's still an interesting read.
I taught my first college class when I was 27 years old, and that wasn't too terribly many years ago. Especially given the nature of the classes I usually teach, it's not unheard of for me to have students who are twice my age or older
None the less, it's established on the first day that I'm to be called "Mr. ...." or "Prof. ..."(I don't have a Ph.D., even though a lot of students do call me Dr. ...). The issue to me is establishing the student/teacher relationship. Initially I was actually uncomfortable with this, but older colleagues advised me that I be firm on this and there is a subtle dynamic shift I've noticed that seems to "reign in" a classroom a bit more.
I'll also add that even though I've had students older than me, the non-traditional students often the most respectful.
I return the respect to my students by referring to them as sir or ma'am, or as Mr. or Ms. so and so.
I often only have students as freshmen, though, just due to the nature of the classes I teach. I'll often deal with the same students in a different role in upper level courses, and I pretty quickly establish with them then that I prefer to be called by my first name. They're no longer my student, and my last name is too much of a mouthful
When I was in graduate school, my advisor made it clear up front that I should refer to him by his first name. My day job is still at the same school where I earned my masters degree, and I call MOST faculty now by their first name. That was only after being directly instructed to do so, however. There certainly is a dynamic shift when you go from being a student(whether graduate or undergraduate) and working along side the faculty as supporting staff where they often need services from you.
Going back, though, I still keep in touch with a lot of teachers and professors to whom I felt especially close. Not too terribly long ago, I called a middle school teacher I hadn't to in a year or so. Even though I've now surpassed her from an education standpoint, she will always remain "Mrs. ..." I'd be uncomfortable with anything else.