Green’s Dictionary of Slang

singleton n.2

[popularized by Helen Fielding in Bridget Jones’s Diary (1996)]

an unmarried person.

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