I reflect on this on my many long hill climbs in the Rockies. I'm 64, and I don't see many graybeards up there, mostly fit youngsters. I'm not passing many other cyclists. And I'm fine with that--I'm still getting up there for some excellent days.
Last week I made my annual 95-mile unsupported trip up Mt Evans, CO from my home in Golden, an 11,000' climb. This is the first year I made it on my first attempt, and it was my shortest time ever. I credit that to picking an excellent weather day, dialing in food and logistics, my head being in the right place, and all the cycling I've been doing during COVID. But I took more breaks above 13,000', and recovery took longer than usual. I don't feel any stronger than at any time in the past. To paraphrase Yogi Berra, 90% of the game is mental. The other half is physical.