Maybe you should pose the question to the 92 year old guy that just set the record for the oldest person to make a Rim-to-Rim hike (N-to-S) of the Grand Canyon.
But he probably had to have the "nursing home shuttle" drop him off at the North Rim trailhead.
https://www.foxnews.com/video/6344326552112
(I did it when I was 60 and in pretty good shape--but I felt those excess 10 pounds every step)