I don't know. I wonder how old Halas was during his last stint with Chicago?
Let's see...
Bud Wilkinson, born Apr. 23, 1916 -- coached St. Louis football Cardinals, 1978-79, went 3-10 in 1979 season...only 63...oh well, I think he was oldest head coach in the league at the time, but obviously not in history.
Marv Levy, born August 3, 1925 -- coached Buffalo Bills, 1986-97 (72).
George Halas, born Feb. 2, 1895, coached Chicago Bears, 1955-67. (72).
So apparently, Halas was the oldest head coach in history.
How about George Allen?
Not sure how old he was when he last coached in the NFL. He was in his eighties (i think) when he coached in college - he died of pneumonia after his players doused him with Gatorade after a big win.