Louis F. Hartman, in Anchor Bible Commentary on Daniel, summarizes background to term 'attîq yômîn ("Ancient One/Ancient of Days).
Louis F. Hartman, The Book of Daniel: A New Translation with Introduction and Commentary (The Anchor Yale Bible Commentary; New York: Yale University Press, 2005), 206
9. an Ancient One. Literally, "one advanced in days" ('attîq yômîn), i.e. an old man; the Hebrew equivalent is bā bayyāmîm (Gen 24:1). Like Akkadian, etēqu, the basic meaning of the root 'tq in Aramaic is "to move forward, to advance"; however, the word yômîn could be dropped after 'attîq, which then by itself came to mean "old."