My wife was saying the same thing a while ago. Saw a neighbor of hers from when she was a child and even though the neighbor insisted that she (my wife) was old enough now that she could call her by her first name, my wife would not, because she's always been "Mrs. Smith".
And speaking of my wife, back when I first started dating her, I was introduced to her nieces and nephews as "Mr. Wheeler". They only started calling me Uncle after we were officially engaged.
I can think of only two elders of mine that I'm on a first name basis with, and that's two of my aunts. The age difference is less than 10 years between me and them. All my cousins also do the same, first name only, not Aunt "Jane". But the older siblings of those two aunts we always used Aunt and Uncle, because it was a much larger age difference.