No max age limits for the POTUS. Besides it's well established that with every published issue, Stan gains 1 extra month of lifespan.
Man, people questioned if Reagan was too old when he was 69 and 73.
Some people thought Sanders and Hillary were too old, and they're 75 and 69. (Trump's 70, but I don't recall if anyone questioned him over his age)
84 might be seen as way too old to pull it off.