Hello Kay,

I think this is all about familiarity. The use of "usted" is for addressing someone you don't know very well, in formal situations or who is elderly. If you know your neighbour fairly well or at least speak to them regularly beyond the courteous "good morning", I would think the use of "tú" would be acceptable. In the same way, they could speak to you using "tú." I speak as an older person and have found that people use "tú' a lot towards me.... and expect it!! 

In some parts of Spain "usted" seems to be used quite rarely. Perhaps that has influenced my view - the teachers will clarify it.

Saludos

John

I am not far short of 80 - and regard it as a compliment when young people (in their twenties) address me as "tú". [This^ sometimes happens - but not always; shop assistants usually tend to use "usted"].

My Spanish teacher [in southeast Spain] says she has never in her life addressed a person directly as "usted" - not even when meeting them for the very first time. [It is difficult, though, to imagine her having done that with the teachers/headmistress in her Primary School. I did ask her what she would do if she met the King or the President, and she admitted that she might have to concentrate and discipline herself if that ever happened - or she might just forget ! ... through not being accustomed to "usted"].

Because of the ambiguity of usted with the third-person he/she, I tend to use "tú" almost always, even on a first encounter. To begin with, I addressed our Alcalde as "usted", but not any more; [he doesn't seem to mind].

Sometimes people at counters address me using “joven”, which is a complement.