I saw your "Ninety is the New Forty" opinion and want to mention Victor Zalgaller who was born in 1920 and has published as recently as 2012. Note his 2004 Annals of Math paper. Of course, Michael Atiyah is 87 and continues publishing, see e.g. his solution of a well known open problem So does John Milnor (86), somewhat less frequently, but see e.g. here (to appear).