In the US, on what occasion do you say "sir" when your listener is obviously younger than you?

Question

I'm Asian who learned English at school. Our teachers taught us in class to add "sir" to be polite when talking to men. I'd assumed that you say so only when your listener is older than you. But after growing up, when I happened to watch an American TV drama (about everyday life) and was surprised to see that this middle-aged man(ordinary office worker, mild-mannered) spoke to a very young man (passerby, looking mild-mannered too) saying "excuse me, sir but..."

I looked up the dictionary and it says in British English "sir" is used by children at school when speaking to. How about the American usage? I now know that irrespective of age gap you say "sir" to a man, but on what occasion do you say so? In what situation?

Answer 1

In American usage it is becoming less and less common to say "sir" and "ma'am," at least in my experience in the Northeast and Midwest. Perhaps retail employees might use it when greeting a customer, and customer support personnel having a telephone conversation might use it to indicate politeness (as body language is not transmitted over the phone). But in normal interactions it comes across as overly stuffy and perhaps even mocking depending on context.

In trying to grab the attention of someone on the street, I would first say "excuse me" when looking straight at someone. I would only say "excuse me, sir" as emphasis to try to get their attention, not necessarily to be polite at all.

And of course in the military it is still used when speaking to a superior at all times.

In all of the above contexts, the respective ages of the speaker and the recipient are nearly irrelevant. Much more important is the power differential: "sir" is used when you are addressing someone of more social standing or power than yourself.

Answer 2

(I'd like to apologize for any false sense of authority that comes off through the formatting of my answer, but I thought the use of headings made it more readable.)

Using "sir" with strangers

I'm about to oversimplify, but as a rule of thumb you can consider it a neutral way to address anyone whose name you don't know. In most interactions with strangers, no one will really care whether you use "sir" or don't, as long as you're courteous overall:

"Excuse me, do you know what time the train leaves?"

"Ten o'clock."

"Thank you."

But it might come in handy if it's the only way you can get their attention.

"Sir? Sir! You dropped your wallet."

Of course, you generally wouldn't use it with someone you actually consider inferior (a small child would be the main justified case of this, but some people are just rude). In cases where perceived social standing is actually in question, it may be a good way to show that you do respect the other person.

Using "sir" with respected individuals

When directed at someone whose name you do know, that, counterintuitively, is when it becomes a term of respect.

With someone you consider a peer, you'd usually use their name once you learn it. By continuing to use "sir", you show that you don't consider yourself worthy of speaking to them as an equal. This is almost always expected in a structured hierarchy like a workplace or military command, but it's also a common way to show genuine, voluntary deference to a respected individual. (If they insist you use their name, though, do it!)

A final note

Apply all the same guidelines to "ma'am"/"miss" when speaking to a female. The only hangup is whether to use "ma'am" or "miss" in a given case, but that's another issue entirely, one you're probably familiar with and can find plenty of resources dedicated to if you don't.

Hope this is helpful, any corrections are welcome.