We really need for all congresscritters to retire at 67 years old.
Term limits: 12 years and out and no defined benefit plan - 401k and NO continuation of any healthcare benefits.
We certainly should make retirement mandatory at 75,for all government jobs, including the SCOTUS, as is required of the pope.
While it is customary for the senator of the majority party with the longest record of continuous service to be elected by the Senate as president pro tempore (or, "president for a time") in the case of the Vice President being absent.
So it doesn't have to be him if the Senate chooses someone else.
The most important thing is TERM LIMITS! Many of these guys have simply been reelected time and time again because they haven’t been really bad, their parties are content to just let things coast along, and most people are too lazy to ask any questions.
Don’t forget, Churchill became Prime Minister at the age of 77. And Donald Trump is now 71. So I think an age limits are far less necessary than term limits.