Even when addressing younger people, I use 'sir', etc... I can't imagine calling them "Sonny"! I think of it as a sign of respect, despite someone's age. I suppose there are adults whom I wouldn't call 'sir' because I don't respect them.
Of course, you are only as old as you feel, yadda, yadda... but I think of it as polite to address elders as 'sir' (unless, of course, they're pricks).
My students call me by my first name and I try to correct them if they slip a 'Mr.' by.
I tried this, but I found that without that initial level of respect (albeit a superficial one), students get too comfortable with me, and the teacher-student relationship becomes more of a friendship, and it's difficult to give a friend a "D" on an essay! Once they finish my class, they are more than welcome to call me by my first name. Interestingly, it is those 'older' students who feel most at ease doing so.
It is interesting, though, that online, when face to face interaction is absent, we feel more comfortable with one another. It seems most likely that it's because we all understand that a forum is not a school or course, but instead just a cafe of sorts where we just happen to share an interest.
Now, if you'll excuse me, ladies and gentlemen, I have to get back to work...:mrgreen: