UK - mid 40's
My DH and I have different family tradition, he comes from an Asian family and is very keen on nieces and nephews calling us Uncle X and Aunty Y. He would always refer to his (now very elderly) aunts and uncles in that way too. I am much less bothered. I don't see my (also very elderly) aunts and uncles much - if I do, I probably would call them Aunty A and Uncle B, or else wouldn't call them anything. If I am writing to them, eg Christmas card,I would always address it to aunt and uncle.
Neither our niece (who is 10) nor our nephew (who is 21) <from different sides, not brother and sister> consistently call us aunt and uncle, although my nephew definitely did when he was younger. My nieces parents (DH's sister and BIL) do not believe in it so it's never happened. I don't mind at all, I think DH does mind quite a lot but isn't going to make an issue of it.
My son is a young adult - he would probably address the relevant people as "aunt and uncle" to their face or in writing, but doesn't if he's speaking about them to us.
When I was a kid,I probably avoided calling my friends parents anything - if I did, it would have been Mr or Mrs Lastname. Not in touch with any of them now but yes, it would seem odd to use first names, at least until invited to. My sons friends, on the other hand, I would find it very awkward to be called Mrs Lastname - I would hate it. Again, they generally don't call me anything, but if they do, it's Firstname (and similarly for my son with his friends parents).