So of the Presidents that have died since WWII ended, 4 died at an age greater than 85 and one is still alive at age >85 (Ford, GHWB, Truman, Hoover, Carter). Three died younger (Nixon, LBJ, Eisenhower) than 85, and one lived past 85 but was effectively incapacitated at by that age (Reagan). One was assassinated. So Biden's probability of making 85 based on just on that sample is about 50/50.
(4 are still alive but not yet 85 - Clinton, Obama, Bush II, Trump).