Btw, you bring up something that has always amused me.
There's all this blubbering about the dinosaurs in Congress, and I agree with it for the most part. But when I first began observing politics around 1980, I had the same question and confusion: "Well then, if they're bothered by older folks holding office, why not replace them with younger ones?"
But the answer to that was always obvious: seniority translates into pork for the district or state. In 1982, John Stennis (81 years old) drubbed Haley Barbour (35 years old) 2-1 in the Mississippi Senate election despite the fact Mississippi was moving red, the Republicans at the time held the Senate, and Barbour later showed how incredibly politically astute he was by compiling one of the best all-around careers of any politician in the state's history (RNC chair, 2-term governor). Stennis was able to make the poorest state in the US quake at, "Well, if Barbour goes to the Senate then Mississippi is gonna lose all their military bases and jobs because seniority." The same thing happened - I mean, he literally ran around my district saying it - with Sonny Montgomery, who would dangle, "If I lose, the Meridian Naval Station and Columbus Air Force Base will close," and he'd win every time.
We've always had those old folks who wouldn't go to the nursing home in Congress. When JFK was murdered, look at the immediate line of succession:
LBJ - 55, which is not old, but he'd suffered a heart attack while in the Senate
John McCormack - Speaker of the House, 71 years old
Carl Hayden - President pro tempore, 86 years old