I'm with Eugene on the aging question (i.e. you have a lot of control over it).
And I'm with Eugene in 3 weeks on the Rim to Rim to Rim, despite being twice his age.
A coincidence? Or not?
First time I went up Whitney, I was 19 and we hiked past a LOT of people and no one hiked past us. Until an 80-year-old ran past us, tagged the top and said,
"The wife is waiting with breakfast at the trailhead", turned around, and ran down.
One of my best days of downhill skiing was a day in Utah when a guy on the lift asked, "Do you want to do some runs together?" and being focused on keeping up with him (late 70s), took my mind off the possibility of falling and I skiied a lot better.
I had the same Chemistry professor at UC Berkeley as my sister, my father, my uncles and MY GRANDMOTHER (she had him in 1917). He was 99 in my frechman chem class. Still doing research, publishing, teaching grad students. He published more after he turned 70 than before. When he turned 70, a UC Regent suggested he go emeritis like most did at 60 or 65. He told that regent he'd retire if that regent could beat him in a quarter mile foot race. Five years later, at age 75, he coached the US Olympic ski team.
Use it or lose it.
I'll grant that I have to start my training, even for a simple 30- or 40-mile day hike weeks before whereas I used to be able to cold-turkey more events.
And, yes, all body parts have a tendency to get bigger, hairier, and closer to the ground.
And I acknowledge that some people get visited by cancer, joint problems, or injuries that just plan suck.
But when Collin Fletcher landed in the ER have being run over by a car, the staff couldn't figure him out. No ID. Face of an 80 year old. Legs and health of 20 year old. Hiking every day of your life is a good thing, IMO.
I went up Half Dome last year and decided to count how many people half my age I could hike past. The answer was "all of them" - something over 200 hikers (Eugene wasn't there).