Plaudits for an essay full of nuance and subtlety! Much of the material I read here on Medium is too simple-minded to be admirable. Your essay is impressive in the number of hands it raises (i.e., "On the other hand...")
Older candidates are, at the gross statistical level, wiser than younger ones, but senators are individuals, not means or medians. Change in our society is accelerating, and the natural conservatism of older people can retard our response to rapid change.
One argument that you didn't raise that deserves our consideration is the fact that anybody who devotes their life to government service necessarily loses touch with life outside of government.
I am shocked to see that some people object to Mr. Kennedy because of his name. There have been only three Kennedys on the national scence in my lifetime. One died 57 years ago; the second died 52 years ago; the third died over ten years ago. Many people alive today don't know a Kennedy from a comedy. It's not a significant issue.