Grandpa rarely walks in, puts his car keys in someone’s hand, and says, “You know, my abilities have started to decline and I think it’s better for everyone if I hand these over.”

So you are saying no maximum age? Let pilots fly until someone takes the keys away? One of my grandfathers had to get into two wrecks (a few weeks apart) before he finally handed them over. One time he fell asleep and hit a guard rail in broad daylight. The second time he hit his neighbors parked car at night.
He consulted in court cases. The guard rail incident happened on his way to work.

My point is, we are terrible at assessing our own abilities. Having a set age is a great thing. The age rule ironically turns 65 this year, so everyone flying has lived with it their whole life. It shouldn’t be a surprise.