It goes against some principles, but I don't think any sitting president should be over the age of 62 IMO. If you're old enough to retire and qualify for SS, well, enjoy your retirement, you've earned it.
Both Biden and Trump really struggled to put together coherent thoughts the last few years - doesn't make them stupid in the capacity of intellect, but it's simply what happens when you're 75+.
My father is one of the smartest people I've known, but when I talk to him on the phone, he's quite forgetful and really needs to focus on what he's saying more. He'll be 76 in November and I wonder how much he has left in the tank.
I fully expect that I too will be yelling at clouds when and if I'm that age, but appearing on live TV in a job that's among the most demanding in the world won't be a place I'll find myself in.