To each his own. I just remember one guy who I flew with in the olden days. He'd strap on 50 lbs of rocks and climb mountains. He'd kayak the Inside Passage in Alaska. When he was forced to retire at Age 60 he was probably in better shape than the new hires coming on the property.
The only difference between the Part 121 pilots and the Part 135 pilots is REGULATION. While our passengers are buying individual seats, their passengers are chartering the entire airplane. So what's the difference between us and the 75 year old Gulfstream pilots passing my jet on the CEPAC routes every day.
None that I can see. Passing a 1st class physical should be the ONLY limitation on a pilot's career.