Over time the average age of Congress has been trending upward (although the House got a bit younger this year).
I suspect two major factors are better health care, which just allows people to live longer, and gerrymandering, which helps politicians win re-election.
But that doesn't explain why so many serve long past the normal age of retirement. I haven't run the numbers, but intuitively it seems to be above the national norm. I see two plausible, and not mutually exclusive explanations.
First, some people just don't want to ever retire. There are entrepreneurial businesspersons, for example, who keep working until they're aged simply because they enjoy working. Possibly, Congressembers are more likely to be drawn from this set of people.
Second, people are likely to continue working longer if they love what they are doing. It's plausible that Congressembers generally love being in Congress. They get lauded regularly, treated like royalty, and get to be at the center of where important things happen. All value is subjective, but power and the trappings that come with it are highly desired by some people.
But there are reasons to be concerned about a gerontocracy in a democracy. While there is much to be said for experience and institutional knowledge and memory, it is also the case that the very elderly can (not inevitably) become less intellectually flexible and nimble, and may not understand the ways in which the world is changing, especially given that they are to a large degree set apart from it.
While I have opposed term limits (because they are largely an effort to limit what voters outside our own state or district do), I would support an age limit on Congress and the presidency. We would exclude some competent people, no doubt, but there is no shortage of people who are as competent for Congress or the presidency as the vast majority of those officeholders have been (which includes the possibility that no human is competent to govern others). I think any harm done would be outweighed by the benefits of eliminating the Thurmonds, the Feinsteins, the McConnells, and the Bidens.