When do you start to experience fatigue if getting out of shape at 58?

Depends. I'm not that far away in age i think it gets harder to get into shape and maintain endurance as we get older but having run nyc marathon at 27 and 54 it can be done- just slower.
Fatigue. Fatigue can start very soon while exercising if you are out of shape. If you are healthy without any major medical problems start on an exercise program and your fatigue will decrease over time as your body becomes more conditioned to the exercise.