Old English ald (Anglian), eald (West Saxon, Kentish) "antique, of ancient origin, belonging to antiquity, primeval; long in existence or use; near the end of the normal span of life; elder, mature, experienced," from Proto-Germanic *althaz "grown up, adult" (source also of Old Frisian ald, Gothic alþeis, Dutch oud, German alt), originally a past-participle stem of a verb meaning "grow, nourish" (compare Gothic alan "to grow up," Old Norse ala "to nourish"), from PIE root *al- (2) "to grow, nourish." The original Old English vowel is preserved in Scots auld, also in alderman. The original comparative and superlative (elder, eldest) are retained in particular uses.
The usual PIE root is *sen- (see senior (adj.)). A few Indo-European languages distinguish words for "old" (vs. young) from words for "old" (vs. new), and some have separate words for aged persons as opposed to old things. Latin senex was used of aged living things, mostly persons, while vetus (literally "having many years") was used of inanimate things. Greek geraios was used mostly of humans; palaios was used mostly of things, of persons only in a derogatory sense. Greek also had arkhaios, literally "belonging to the beginning," which parallels French ancien, used mostly with reference to things "of former times."
Old English also had fyrn "ancient," which is related to Old English feor "far, distant" (see far, and compare Gothic fairneis, Old Norse forn "old, of old, of former times," Old High German firni "old, experienced").
Meaning "of a specified age" (three days old) is from late Old English. Sense of "pertaining to or characteristic of the earlier or earliest of two or more stages of development or periods of time" is from late Old English. As an intensive, "great, high," mid-15c., now only following another adjective (gay old time, good old Charlie Brown). As a noun, "those who are old," 12c. Of old "of old times" is from late 14c.
Old age "period of life of advanced years" is from early 14c. Old Testament is attested from mid-14c. (in late Old English it was old law). Old lady "wife, mother" is attested from c. 1775 (but compare Old English seo ealde hlæfdige "the queen dowager"). Old man "man who has lived long" is from late Old English; the sense of "husband, father, boss" is from 1854, earlier (1830) it was military slang for "commanding officer;" old boy as a familiar form of address is by c. 1600. Old days "former times" is from late Old English; good old days, "former times conceived as better than the present," sometimes ironic, is by 1670s. Old Light (adj.), in religion, "favoring the old faith or principles," is by 1819.