Not true for me but then again I never respected adults
I never called my parents by their first name out of respect but I was never told I couldn't call another adult by their first name. In fact I use to sit with an elderly lady when I was younger that she would get on to me if I said Mrs. Smith, I was to call her Laura.
I raised my kids to say "Yes, Ma'am / Sir" and "No, Ma'am /Sir" just like I was but I ain't about to get on to them for calling an adult by their first name.
Most of the adults in my neighborhood wanted to be called by their first names. So that is what I did.
I never knew most of my friends parents names...
Of course not! It's called 'respect'.
For me it's not polite to adress anyone significantly older than me by first name ever.
It depends, on the person whom you call them.
That's right. And I expect to be addressed that way today.