Hi! 1) contra the first answer: "eldest" can be used ONLY of people, but "oldest" can be used for anything (people, animals, things) 2) "eldest" may be used to emphasize rank or seniority ("oldest" does not do this), and need not be emphasizing age 3) "eldest" is used in comparing a group ("my eldest son"/"the eldest of my three sons"), but "oldest" can be used in a more absolute sense ("the oldest man alive")
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