late 13c., from Latin senior "older," comparative of senex (genitive senis) "old," from PIE root *sen- "old." Original use in English was as an addition to a personal name indicating "the father" when father and son had the same name; meaning "higher in rank, longer in service" first recorded 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 first recorded 1938, American English.