I think you need to think about how much older the person is. In the middle of the U.S.,  if you are 20 and the person is up to 50 years old, if it is an informal situation, I think using their first name generally works fine.  If by older person, you mean someone over 65, you probably should not use their first name unless they invite you to do so. Obviously there is a gray area, depending on how old you are and how old the other person is. 

You should also be much more careful about using first names in America if the person is a racial minority and you are not. As SHL indicated, older black Americans often were aware of being treated disrespectfully being called by their first names and see it as very disrespectful. When I was in college during the 80´s, a social work professor assigned us to do taped interviews with older black people about the racism they had experienced over the year for our Black Historical Museum.  She warned us that any use of their first name would be given a failing grade.  I have been cautious ever since-- but gradually learned that black people my age or younger (I am now in my fifties)  often expect to be called by their first names by me and laugh when I tell them why I was calling them ¨Mr ....¨ and ¨Mrs ....¨

The thing is that every culture and sub-culture has their own rules-- so no one can give you a very good answer about this.   

The standard in the US is never to call an older person by his or her first name unless they invite you to or the person is a close relative or friend already. It´s considered rude and disrespectful. Sir and Ma`a are regional and are ok in the US Southeastern States only (Alabama, Mississippi, Georgia), but those terms are appropriate anywhere in the US  in a store or restaurant. Otherwise, you need to say Mr. or Ms. 

There was a time, say pre-1960s, where in the US South (again, Alabama, Mississippi, Georgia) black people were always addressed by their first names, regardless of age, but the black person had to address the white person as Sir or Ma`am or by the person`s last name, or risk being murdered (I`m not kidding. Literally hanged). It was extremely dangerous for black people in the US South in those days and these sorts of murders were never investigated or prosecuted by local police. The police just looked the other way in those days. 

In a courtroom in the US South, a black person would be addressed by the Judge by his or her first name, like „Billy, now tell us what you saw.“ It didn’t matter how old the person was. But a white person was referred to as sir or maám or by his or her last name. It was the equivalent of Aparteid South Africa back then. 

It may be this legacy that has caused the aversion to addressing older people informally in the US. I don’t know. 

@Bob

I don`t think the use of sir is really the question. Sir is used when you don’t know the person`s name or when you are serving someone in a restaurant or business. The question is more, what if you do know the person`s first name, when do you address him with it, or use Mr.?

Wait in line and be looking at your phone and you might here "excuse me Sir but I think you're next". Go into a hotel and the reception will often say: "May I help you sir". Sir is used many ways in the US.