Here, according to Wikipedia's statistics, and with apologies to the ladies, are the top twenty oldest performers who regularly grace the stages of Las Vegas (or, in the case of a few, are iconic stars associated with the city, even if they seldom perform here these days):
Marty Allen: 89
BB King: 85
Jack Jones: 73
Tom Jones: 71
Paul Anka: 70
Bill Medley: 70
Wayne Newton: 69
Barry Manilow: 68
Gladys Knight: 67
Rod Stewart: 66
Clint Holmes: 65
Elton John: 64
Kenny Loggins: 63
Teller: 63 (Penn's a mere 56)
Peter Noone: 63
Rick Springfield: 61
Charo: 60
George Wallace: 59
Louie Anderson: 58
Rita Rudner: 57
UPDATE: 08-08-2011 A few readers have written in with suggestions for other possible inclusions to this list, including Don Rickles, who does still pop up from time to time (85), Englebert Humperdinck (75), and Jay Leno (61).