I am a native English Speaker (grew up in Ohio and have lived in Minnesota for 34 yrs.) "An elder" refers to an old person but I could say "an elder brother lives in Texas" meaning "an older brother". It's just that you are MORE likely to hear "an older brother". as TeribleT says, it's not used often. The "eldest" sibling is still used. I sure wish I had some friends to answer my Hebrew questions :)
You misread your own source. When it says "they also tend to connote seniority" it is telling you 'elder' tends to be used as a comparative. For example, to say the brother who is older than I, "my elder brother". I can say my elder brother when I am 10 and he is 12. This has little to do with "he's a senior or elderly". Re-read the source you quoted. BTW you are good to look for sources like this. English is a broad language with many meaning we don't experience in our own lives. This is why I always double check my grammar before posting, and why I appreciate that you did too.
TeribleT, Clearly you are right if you say that elder has other meanings besides older. It can be a tree for example. Where I disagree is your assertion that elder is no longer common usage usage for the meaning "older". Both dictionary.com and https://dictionary.cambridge.org/grammar/british-grammar/elder-eldest-or-older-oldest do not say this. I don't see your source claiming this either.
'Elder' has other meanings, in addition to being a form of 'older'. For example, it's also a church title. See https://en.m.wikipedia.org/wiki/Elder_(administrative_title). Here's the first paragraph of that entry: "The term Elder, or its equivalent in another language, is used in several countries and organizations to indicate a position of authority. This usage is usually derived from the notion that the oldest members of any given group are the wisest, and are thus the most qualified to rule, provide counsel or serve the said group in some other capacity."
Very interesting discussion about eldest, elder, oldest and elderly. I heard each one of these, though lived in USA not all my life but less than 10 years. Can anyone from you wise and hebrew-speaking people tell me why the verb שומר, which is to "guard" is used here as "watch over me", as in the other languages l speak the terms used are "watch" or "take care", does this have a cultural connotation in Hebrew or have to do with the language construction?