The first thing you need to know early on about "age" is that you should be thinking in terms of age brackets. Before we go into the nuances of the next paragraph, you should not be worrying about absolute age, but rather what I call "general" age; is the other person much older, much younger, or about the same age as me. In that case, you should tailor your forms of address to "older," "younger," or "same." In most cases, your judgments should be reasonably accurate, although you may have to fine tune.
Later on, when you are among your "peers," they may treat each other as such, or there may develop an informal hierarchy among you. If the latter is the case, you will likely be informed as to where you stand in the group and be able to act accordingly. If not, ask the friend that you are closest to, where you stand in relationship to him/her, or better yet, one or more third parties.
As a Westerner, you need to "try" to fit in, although you will be given some leeway for your inevitable mistakes.
"Korean," particularly Korean-American communities can vary greatly Some are formal, others less so. The fact that the community is so "sticky" about hierarchy while not even speaking Korean makes it rather unusual on this regard. This is particularly true when the non-use of the Korean language makes it sound like an expatriate community (which the edit to your question now makes clear). Most communities (not all Korean) that are so "orthodox" in one way are so pretty much across the board, including language, dress, and customs. Frankly, I've never heard of such an age hierarchy being administered in English or other "non-native" language, (although i guess such a thing is possible).
In a sense, our exchange has been like the story of the six (two) blind men and the elephant. I don't claim to understand the whole (Korean) "elephant," only the part that I'm holding onto. Which is to say the Korean-Americans (and a handful of native Koreans) that I know in America.