Green’s Dictionary of Slang

an unmarried person.

1961		Missoulian (MT) 28 Mar. 9/5: Junior Singletons — young unmarrieds under 45 [...] Senior Singletons — unmarrieds over 45.
1999		Indep. Rev. 18 June 11: A fortysomething singleton who wards off loneliness by picking up men at local caffs.
2000		Guardian Weekend 8 Jan. 11: Bridget Jones and her fellow singletons.