Green’s Dictionary of Slang

of age, within a specified decade, e.g. thirty-something; thus used in pl. to categorize a generation, thirty-somethings, twenty-somethings.

1985		O. Hawkins Chili 22: If she’s around today, she has to be a plumpish 40 something odd.
1988		Newsweek 12 Dec. 50: The health conscious ‘thirtysomething generation’ has begun to shy away from fat-and-sodium laden fastfood fare.
1989		Wall Street Journal A 5 July 9: A fortysomething group.
1999		Eve. Standard 4 June 54: She’s an experienced thirtysomething.
2000		Indep. Rev. 21 Jan. 7: You can watch amiable twentysomethings in casual clothes tapping away at their work-stations.
2005		L. Theroux Call of the Weird (2006) 192: Dark-skinned and almond-eyed, twenty-something, his hair in a do-rag.
2007		L. Redhead Cherry Pie [ebook] I wondered why all the inner-city thirty-somethings hadn’t discovered its ironic joys,.
2010		Codella and Bennett Alphaville (2011) 307: A thirty-something black guy who looked like he’d been sleeping outdoors.