When I was a teenager, I got to meet the famously old-but-still-in-office Strom Thurmond. It was a couple of years before he retired from the Senate, so he would have been around 98 at the time. We all lined up to shake his hand. He still had a solid handshake at that age, but one look in his eyes and you could see that he was mentally long gone. He greeted each person with the exact same words, spoken in a mindless monotone. It felt crazy to me that he was still reelected one more time after that.
Sidenote: He wasn't just famous for holding the record of the oldest serving US senator, he also held the record for the longest solo filibuster, holding the floor for over 24 consecutive hours in opposition of the civil rights act. He hated taking away the states' right to make racism the law so much that he led the defection of all the racists from the Democrats, who, outside of the south, were really starting to get on board with the whole civil rights thing, over to the Republicans, who welcomed them, setting the stage for current party lines on the topic.