I don't know how long I can manage, but in my mind I'm just getting started. Casey Stengel went to 75. If I decide to stay, I won't turn 75 until the 2006 season, so I have two more seasons to go to tie Casey. I don't think I can catch Connie Mack. He went all the way until 88. I don't think I want to do that.
-- Jack McKeon, I'm Just Getting Started (2005)
McKeon didn't "tie Casey" ... not then, anyway. He left the Marlins after the 2005 season -- the club's second straight 83-79 campaign -- when he was still only 74.
Those comparisons really aren't all that elucidating, though. Stengel's last four managerial seasons came with the Mets, when they were terrible and, by most accounts, Casey wasn't always paying a great deal of attention. Mack, by most accounts, assigned most of his managerial duties, beginning in the early 1940s, when he was around 80, first to his son Earle and later to coach Al Simmons. Mack remained as the official manager until 1950, when he was 88, only because he also owned the Athletics and apparently had trouble imagining anyone else manage the Athletics.
McKeon thus might be considered the first real 80-year-old manager, in the sense that he's being hired by someone else (as opposed to Mack) to actually win baseball games (as opposed to Stengel).
Will McKeon turn the Marlins around? Well, they've lost 10 straight games; at this point, a lawn jockey would probably turn them around. Remember, McKeon's last stint wasn't that long ago; he took over in May of 2003 when he was 72, somewhat miraculously drove the Marlins to a World's Championship, and subsequently managed them to a couple of winning seasons before retiring (temporarily, as it's turned out).
Of course, when he took over in 2003, we all thought he was too old. Only Stengel and Mack had been older managers, and neither had been successful. Since then, Joe Torre's managed the Dodgers to a division title at 68 and Tony La Russa's still going strong at 66. So while the notion had generally been that managing was a middle-aged man's game, maybe we should redefine "middle age" (I know I certainly have, now that I'm technically there).