(Internet, slang, offensive) An oldbie; a veteran member of the 4chan community (or any other online community).
2009, 18 June, Onslaught Six username, Re: I really didn't study for this test, The pathetic Usenet oldfags will cling to their pathetic, outdated and boring medium as the new-world forumites rule the Interwebs universe.
2011, Cole Stryker, Epic Win for Anonymous: How 4chan's Army Conquered the Web, The Overlook Press (2011), ISBN 9781590207383, unnumbered page:Oldfags will post the symbol along with "Newfags can't triforce."
2012, Parmy Olson, We Are Anonymous: Inside the Hacker World of LulzSec, Anonymous, and the Global Cyber Insurgency, Little, Brown and Company (2012), ISBN 9780316213530, unnumbered page:There were many like him—oldfags who believed they were the true Anons.