I've lived in TN all my life and it's practically ingrained in me to say Ma'am/Sir to EVERYBODY. If I'm not sure that you are younger than me, I say ma'am or sir. Older people (50+) get called ma'am/sir all the time, but, with other age groups, the title can fall off if you get to know them really well. When I go through the drive through I say it ("Would you like fries with that?" "No ma'am"), whenever somebody asks me a question, it's normally a yes or no followed by ma'am or sir, it's gotten to be a habit. And it's a peeve of mine when people get upset when you call them that, for me, it's not about age, it's about respect. I almost got into a brawl in New York when this older lady got mad at me for calling her ma'am!
I just wish people wouldn't be so sensitive.