When we address someone we don't know, we will generally be more formal when addressing them, e.g. "Excuse me, Sir/Ma'am." When the person is our boss/superior it is up to them to decide how they want to be addressed, some prefer Mr/Mrs/Miss, others prefer first names, it varies on a case by case basis.
When meeting someone for the first time, we always start out with the formal address and wait for them to tell us what their preference is, or else we ask them directly. We also take our cue from how they are introduced to us. If they are introduced as "Mr. John Doe", we would call them "Mr. Doe" until told otherwise. But if they are introduced as "John" then that is what we would call them. It would generally be impolite to insist on calling them "Mr. Doe" if they have been introduced to us as "John". But this depends somewhat on the specific circumstance.
Of course there are also many subcultures within America who each have their own way of doing things when speaking to other members of their subculture. If you are not a member of that subculture it would be very wrong for you to copy their manner of addressing each other.
In the last 60 years there has been an observable change in our spoken language that is not very strongly reflected in written language. American English used to be a lot more formal, when children through to young adults addressed older adults it was always with Mr/Mrs/Miss (surname), at least that is how I experienced it growing up (on both coasts). I was quite surprised to discover about ten to fifteen years ago that it is now common for even young children to address adults by their first name. This would have been unthinkable when I was a child. So yes, American English/Society today is less formal than it was 60 years ago.
IMHO Strongly stratified class structures such as those that exist in India are an impediment to communication and progress. Perhaps that is why English has become the primary language of technological innovation. As others have pointed out, being informal is not the same thing as being disrespectful. All this honorific stuff typically degenerates into pretentiousness and arrogance in which people are afraid to express their opinions and insights for fear of offending the alpha dog.
As a matter of fact it can be observed that in India all of this so-called politeness actually has the opposite effect. Sure, everyone is polite to the top dog, out of necessity/convention, regardless of how they actually feel about them. But the top dog is essentially obligated to be rude in return by making sure that the people socially beneath them know that they are the lesser. Groveling is expected. In India you have an entire class of people called the "untouchables", who are defined that way by happenstance of birth. You are in essence obligated to be rude (or worse) to these people for no rational reason at all. In ~contemporary~ America no such concept would be tolerated. How then can you say that English is inherently "rude" but Hindi is not? The fundamental assumption behind this question is actually incorrect, you have rudeness built directly into your language. Yes, you feign respect to your superiors but you also are obligated to express disdain for your inferiors, and this is inherent in the very language constructs by which you address each other.
In English, we also make frequent use of tone of voice to convey respect; without the need for special/specific words to express it. When we give respect to someone, it is not mandated by a convention of language structure, and usually not by social status (there are exceptions), instead it is genuinely heartfelt and comes across in the softness and manner with which we speak. A "respectful tone of voice" is a tangible thing. It is easily identified when heard, but perhaps is not possible to describe in writing. It is not so much in what we say as it is in how we say it.
I think that there is a huge and important difference between pretend respect demanded by language conventions and actual respect which has been earned in some way. Because of the lack of obligation imposed by the language structure, English is more honest in this regard. Perhaps somewhat paradoxically, being honest towards someone is actually far more respectful to them, even when you don't respect them, then it is to pretend to have respect for them when it is not genuine.
Another way we show respect when someone has earned something by merit, is that we will often refer to them by title -- if they have earned it -- such as "doctor" or "professor", rather than saying "hey you" :-). Sometimes this title is honorary rather than actual, for instance someone who's intelligence is highly regarded may be called "the professor" even though they are not actually certified as such. The term "doctor" or "doc" can also be very confusing, because it usually implies a Medical Doctor (MD), but the term actually applies to anyone with a PhD regardless of what subject they earned it in.
In American culture we do not assume that simply because someone is old that they are automatically entitled to special respect. We are far more interested in what someone has accomplished then we are in how old they are. Perhaps this came about because of changes in life expectancy. When the average life span was 30 years old as it was in the pioneer days, anyone who made it to 60 or beyond, did in fact accomplish something special.
But these days when the average life expectancy of everyone is well beyond 60 years, just the fact of being old is no longer anything special. Also older people tend to hit a point at which they become progressively less able, especially mentally, than they were when younger. Someone with diminished mental capacity is not going to be accorded the same level of respect that they had when they were more able. Furthermore, an old scoundrel is still a scoundrel and just by growing old they still have not done anything to merit being respected. What you have done with your life is far more important than how old you are.
