I used to go to this restaurant for take out about once a week, and the guy at the counter would always call me by my first name multiple times per visit: "Hi Dave", "What can I get you Dave", "That'll be $7.50 Dave", "Here's your food Dave", "Have a good day, Dave".
I always paid cash, but since I was at work I had my work ID clipped to my belt loop and he saw it once and just capitalized on it. Extremely annoying.
What was he, Hal from 2001? That does sound annoying.
As for the whole addressing your elders by Mr or Mrs., for me it depends on the circumstance. If you're talking to a teacher or professor, than that is acceptable, but anyone else I might say sir or just call them by their first name.