I have alot of trouble calling my friend's parents by their first names as *I* feel that it's rude...but of course if they tell me they prefer it that way I get around it by...
only speaking to them when they're looking at me.
I just *can't* do it, it feels wrong and makes me feel uncomfortable to call someone by their first name without having some sort of friendship with them personally. Even for kids my age I always wait until they've introduced themselves to me (even if someone else has already told me their name) before I use their first name to talk with them.