oldest; esp., firstborn or oldest surviving
Origin of eldestMiddle English from Old English eldest(a), ieldest(a), superlative of ald, eald: see old
Greatest in age or seniority. See Usage Note at elder1.
Origin of eldestMiddle English from Old English eldesta ; see al-2 in Indo-European roots.
The normal superlative of old is oldest. The irregular form eldest is sometimes used with family members, but it is otherwise rare.