>>...first time a stranger young person address you using “usted”...<<

The Afrikaans have a similar custom: the word "Oom" means "Uncle", and "Tannie" means "Aunt". Younger people use it as a mark of respect, regardless of the relationship to whomever is being addressed. I was nineteen when I was first called "Oom". I didn't get over it until years later when my first nephew was born, and I felt I had earned the title!

Wish I knew a way to "mark-up" those words to convey the sounds!