What does the phrase "an old friend" mean?
What does the adjective "old" mean in this case?
It seems to me this phrase refers to a friend we have known for a long time rather than a friend who is long-lived.
If this is the case, what word should we use to refer to a friend who is not young?
Thank you very much in advance!

Question

Answer

You are 100% correct!
"an old friend" refers to a friend you have known for a long time, not a friend who is actually old.

Answer

You could say: My elderly good friend...or my elderly friend... 
By saying this, there is no confusion, because you are only saying that your friend is old of age but not saying that you have known him for a while like you would if you said "My old friend". Or if you really want to be poetic :) you could say: " My dear friend, who is old of age but not of soul, ....."
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