"Senior citizen" was and is also used in New York City. I have a 1917 "senior citizen" citation that refers to New York lawyer Joseph Choate, but this is best regarded as an isolated use of the term.
(Oxford English Dictionary)
senior citizen, a term for an elderly person, esp. one who is past the age of retirement. orig. U.S.
Freq. used in official communications and by the media as a euphemism for 'old-age pensioner'.
1938 Time 24 Oct. 12/2 Mr. Downey had an inspiration to do something on behalf of what he calls, for campaign purposes, 'our senior citizens'.
28 February 1917, Mansfield News, pg. 11, col. 1:
PAY TRIBUTE TO
JOSEPH H. CHOATE
Aorse From Sickbed to Cele-
brate Eighty-fifth Birthday.
A LEADER OF LEADERS.
Joseph H. Choate, dean of American lawyers and senior citizen of the United States, arose from a sickbed the other day to celebrate his eighty-fifth birthday.
18 February 1929, Appleton (WI) Post-Crescent, pg. 10, col. 4:
The Senior Citizen's association of the Columbus school met at the schoolhouse Friday evening.
15 April 1937, Los Angeles Times, "New Pension Plan Offered," pg. 5:
SACRAMENTO, April 14. (Exclusive) - Robert Noble of Hollywood took the rostrum in the Assembly today and addressed members of the Legislature on his "roperty (sic) certificates" for senior citizens of California.
4 July 1938, Los Angeles Times, letters, pg. A4:
Every "senior citizen" who would receive his "$30 every Thursday" would spend the warrants as quickly as possible to avoid having to attach the weekly 2-cent stamp required.
25 September 1938, Los Angeles Times, pg. A4:
It will be over the determined protest of John Taxpayer, indeed, that either the "senior citizens," or those who fell for their optimism currency, collect even the full amount per ticket represented by the stamps licked and stuck thereon.
28 October 1938, Los Angeles Times, "The Great Game of Politics" by Frank R. Kent, pg. 7:
WASHINGTON, Oct. 27. - One of the major developments in American politics, frequently commented on in recent months, is the multiplicity of new schemes for granting and increasing pensions to the aged - or, as some politicans are tenderly beginning to call them, "our senior citizens."
31 July 1938, Washington Post, pg. B9:
The build-up is that every "senior citizen" has done his share of the world's work and is now entitled to a living.
20 September 1938, Reno (NV) Evening Gazette, pg. 11, col. 2:
...to solve unemployment, to end the need of doles, to care for senior citizens - retirement life payments is proposed to California as state constitutional amendment.