In his farewell address, Washington warned against the dangers of concentrated power and the potential for tyranny if a president were to remain in office for too long. Washington was of course referring to the presidency, but the same rationale applies to all elected officials.
Since the late 1800s, once elected to Congress you have a 90% chance of winning re-election.
I’m in favor of an amendment that caps the age for running for any federal office. Older than 75 seems like a good spot. Potentially you could serve in the Senate until reaching 81. Seems like a good place to cap the age of federal judges as well.