"aged (some number of years)," abbreviation of Latin aetatis "of the age of," genitive singular of aetas "age" (see age (n.)). "Chiefly used in classic or scholarly epitaphs or obituaries" [Century Dictionary].
"former ages, old times," c. 1400, poetic or archaic form of old; in some cases from Old English eald, yldu, yldo "old age; an age; age as a period of life."
1660s, "suited to or characteristic of old age," from French sénile (16c.), from Latin senilis "of old age," from senex (genitive senis) "old, old man" (from PIE root *sen- "old"). The meaning "infirm from age; pertaining to or proceeding from the weakness that usually attends old age" is attested by 1882; senile dementia is attested by 1851.