Old English ald (Anglian), eald (West Saxon) "aged, antique, primeval; elder, 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 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; Greek 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," 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"). 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.
First record of old-timer is from 1860. Expression old as the hills is recorded by 1819. The good old days dates from 1828. Of old "of old times" is from late 14c. Old Testament is attested from mid-14c.
Old maid "woman who remains single well beyond the usual marrying age" is from 1520s; the card game is attested by that name from 1844. 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 c. 1200; sense of "husband, father, boss" is from 1854, earlier (1830) it was military slang for "commanding officer;" old boy originally was a former pupil of one of the English public schools.
Old English is attested from 1701 as a typeface. It was used to meaning "the Anglo-Saxon language before the Conquest" in a c. 1200 account of the native (as opposed to Latin) month names, but the modern linguistic use is from 19c. (see Middle English).