At 77 I don't get any more tired than I did when I was 30. The reason: I'm more willing to spend on business (or at least real PE) tickets. I can take an extra day or two to rest up and get used to a time change before getting into serious activities. I take the time/spend the money to treat myself better in every other way. I'd probably get more exhausted if I tried to maintain the same schedule and traveling conditions as I did then, but I don't have to - so I don't.