And you'd get shot in most 'Latin' countries for it!!!
I speak around fourteen different languages to a greater or lesser degree of profficiency. With almost NO exceptions, most languages have a polite form and a familiar form. English, by contrast, has the same words for each form, which makes learning English easier for foreigners on the one hand - there are less words to learn - but more difficult in some ways. The French, German and Spanish speakers simply can't understand why we do not have separate words for polite and familiar forms. Using the wrong form is very bad, er, form!
"Du" in Germany may be acceptable amongst family or close friends, or from an elder to a youngster, but in business and in most social circles, only "Sie" is considered acceptable.
Likewise in French, 'tu' is the familiar form and 'vous' is the polite form. The two are simply NOT interchangeable. Not only are the terms of address different, but the verb conjugations are also different, so it is harder to accidentally make a mistake.
When to use the formal or the familiar also depends on context as well as a knowledge of etiquette. This applies equally well to English (in England) as it does to other languages.
My father, for instance, is my 'Old Man' in everyday usage. "How's your old man doing?" or "Did your old man get to the Southampton game last week?". I would not, however, and neither would most children call him that to his face, and a father-son relationship is about as familiar as you can get!
Old Man is also another 'familiar term' for husband. "My old man spends all his time on the computer" is a familiar complaint, but no wife (who wanted to remain married!!) would use it as a direct form of address.
Between friends however it is a different story...especially amongst the middle and upper classes. "How the devil are you, Old Man?" or "I say, Old Chap, did you invest in....". Old in this case refers to a friendship or acquaintance of long standing, not a reference to the age of a person.
Referring to people by first names as a matter of course is a very bad show indeed. Somewhere like this forum, where we identify ourselves by first name or alias it is acceptable. Between colleagues at work it is acceptable. Between strangers, it is NOT acceptable (in the UK and most of the rest of Europe and the Latin World) UNLESS you have been introduced by a third party by first name, or you have introduced yourself by first name....or the person with whom you are conversing has made it clear that first names are acceptable. The expression 'on first name terms', meaning to know someone well has derrived from this very point of etiquette.
Now in this modern and lax age we live in, politeness is rapidly becoming obsolete. I for one choose not to take offence if someone DOES address me as Mike instead of Mr. Percival when the polite form is more appropriate, but there are those who do consider it very impolite. I was always taught to address strangers or my elders by surname until I was invited otherwise. One or two offspring of my colleages still to this day use the polite form when they see me, and they are now in their late teens and have known me for years. I even said to one that they COULD call me Mike, but THEY demurred stating that as I worked with their father it didn't seem right to them! Other children of colleages have always addressed me as Mike (or Uncle Mike) because I have introduced myself to them as such.
Now, if my bank manager addressed me by my first name, I'd be upset. He hardly knows me and as far as he's concerned I'm just a name and a number. I don't even KNOW his first name, just his initials. Counter staff refer to me by surname, even though I went to school with a few of them, they still have to call me 'Sir' or Mr Percival in earshot of their seniors!! But, they wear name tags and it IS acceptable to address THEM by first name.
Funny old world, innit?!!!
