Years ago an old riding buddy in his 70s, at the instance of his wife, gave it up after several crashes/falls that were of his own doing. The last one occurred while walking down a hill in Prague. Simply lost his balance, fell down the hill and broke several bones. His doc told him his next fall could be his last. He took to taking indoor training classes and walking on flat ground.
On the flip side, I once rode unsupported across the country with a group of 12 others. The oldest guy in the group turned 77 during the trip. After we finished the tour he started riding home until is wife made him stop. Ultimately, he lived to be 91. Don't know how long he kept riding, but I suspect it was a while. Guy was a tough bird who spent two years in a Nazi P.O.W. camp as a member of the RCAF before eventually becoming an American citizen.
We all lose our "faculties" at different ages and at different rates.