late 13c., "the elder," from Latin senior "older," comparative of senex (genitive senis) "old" (from PIE root *sen- "old"). Its original use in English was as an addition to a personal name indicating "the father" when father and son have the same name (e.g. Walterus Baddyng, seniore in late 13c. Leet rolls of the City of Norwich). The meaning "higher in rank, longer in service" is recorded by 1510s.
The Latin word yielded titles of respect in many languages, such as French sire, Spanish señor, Portuguese senhor, Italian signor. Also compare Herr. Senior citizen "elderly person" (typically one past retirement age) is by 1938, American English.
mid-14c., "person of authority;" late 14c., "person who is older than another," from senior (adj.). Sense of "older fellow of a college; fourth-year student" is by 1888, from an earlier general sense of "advanced student" (1610s). The meaning "aged person, one of the older inhabitants" is by 1889.