I still cannot call adults by their first name even tho I am older than them. You just NEVER did so when I was growing up.
I have to admit, I love a "Southernism" that I grew up with. Adults you are close to can be addressed by their first names, but always preceded by "Mr." or "Miz" (whichever is appropriate). It has some of the intimacy of using the first name, but is still respectful.