We don't use honorifics like this in English. They're common in Chinese and Japanese and (I think) Korean but we don't use them in English.
You'd be more likely to find an upperclassman harassing or hazing a freshman and calling them "freshman" as an epithet.
Generally in the US, the years of compulsory education are referred to by grade number up to grade eight and by the terms freshman, sophomore, junior and senior for grades 9-12, respectively (though some school systems limit high school to grades 10-12).
Regardless, if one wishes to say that another person is in a year other than their own, they would say:
He's a [freshman/sophomore/junior/senior].
She's in 4th grade (etc).
He's [in] a grade above me.
She's an upperclassman.
He's older than me.
So, for something like:
Hey, 学长/学姐, can you tell me how to get to the cafeteria?
One would simply say:
Hey, can you tell me how to get to the cafeteria?
Where's the cafeteria?
Honestly, at a public school, I don't think I could usually tell what year someone was just by looking at them, so I'm not sure how one could always know they were correctly addressing a senpai (the Japanese version of 学长/学姐), particularly in a school of 3000+ teens. I tried to find some info about the average school size in China but nothing obvious turned up for me.
Also remember that the system of schooling in the US is vastly different. It's not uncommon to be in a single course with every grade. For example, when I took a painting class in high school, there were members from all four levels in the same class, and no one really cared what grade you were in.
The address is whatever magic words will get mail delivered to you.
Normally the first part of the address is your name. So something like
Danny Kim, Seat 111, Floor 4
ABC Center
1234 N ABC Rd
Washington DC 12345
Should probably be enough information to find you. Generally, you put more specific information first.
However there is an easy way to check... talk to the person in seat 110 or 112 about the mail system in your building. It may be that mail is held centrally and you need to pick it up, so including a seat number is pointless (though it wouldn't cause the mail to be rejected)
Best Answer
Nowadays, social protocols regarding forms of address in 'polite society' in western English-speaking countries are much more relaxed than they were 100 or 200 years ago, especially in the USA (and even in stuffy old Britain).
Unless you're part of the Hollywood crowd or a London West End actor, most people would probably reserve 'Darling' for people with whom they have a romantic relationship. 'Dear' also implies a certain degree of (at least emotional) intimacy.
But I don't think age differences are much of a factor now in terms of the general expectations that people have about how they ought to address one another.
The chief exception is perhaps such limited circumstances as where a large status difference is coupled with an occasion of some formality, especially when it also involves an official hierarchy with its associated rules of proper conduct (such as pertains to a branch of the armed forces).