Robert Byrd was born in November 1917, and died on June 28, 2010. That made him 92 years old. He was also the longest serving member of the United States Senate. Senator Byrd served uninterruptedly for 50 years. That’s an impressive record of public service, well. He was also a member of the KKK but as far as we can tell he had reformed.
He was older than John McCain, before his death.
The United States President has control of American Nuclear weapons. He may have to make a fast decision without having time to consult advisers in an emotionally charged situation. A terrorist disaster killing the three people who should have been president before Byrd was unlikely but possible. Senator Byrd was entitled to stay in the Senate as long as he wanted to and as long as his constituents had confidence in him. There was a strong case for Byrd giving up the position which could have lead to the presidency.
Trump in charge of nuclear weapons is even more frightening.