I'd say 50-65 would have to be the sweet spot for senior government officials. anything younger would be lacking in exp. & wisdom. anything older would raise concerns about health & mental condition. and don't even get me started on "for life" appointments (SCOTUS, for example) that's just plain idiotic...
I'm 62 and still learning. The idea that in three years I will fall off a mental cliff and become an idiot seems outlandish to me.
Yes, dementia exists, but the voters should be able to detect and prune demented officials. There's no reason not to have a leader who is 90 if he is still visibly functional.
“Life is a shipwreck, but we must not forget to sing in the lifeboats.”
― Voltaire