Well, if we want to split hairs (and heck, it's an internet forum - that's what they're for):
Olson never coached a game as a 73 year old. He missed the entire 07-08 season and he retired right before the 08-09 regular season started. (birthday: September 22, 1934 Final game: March 16th 2007) = 72 years, 5 months, 23 days
Chaney was 74 years, 1 month, 22 days when he coached his last game (birthday: January 21, 1932 Final game: March 14th, 2006) = 74 years, 1 month, 22 days
Jim Phelan was almost 74. (birthday: March 19, 1929 Final game: March 1st, 2003) = 73 years, 11 months, and 11 days
Boeheim turns 74 this coming November, so on January 8 2019, he will become the oldest D1 basketball coach ever (Birthday: November 17, 1944 Final game: TBD, age on January 8 2019) = 74 years, 1 month, 23 days
My math might be wrong, so feel free to check it.