A gerontocracy is a government run by old people.
Societies are usually run by people in the second half of their lives, people age 40 to 60. That's normal and healthy. Wisdom and empathy both come from experience. You cross the line into gerontocracy when power concentrates into people in their 70s and beyond.
Historically, gerontocracy has not gone well. The USSR in the early 1980s, China in the 1900s, Austria heading into WW1: if you look around you and the people in charge are all very old, expect turbulence.
The reasons from this range from the poetic to the actuarial.
First, gerontocracy represents a failure of imagination. Time horizons become compressed. It's rare for an 80 year old to start a brand new project. People at that age naturally want to complete some vision. Whatever dissonance they still feel in their own story, they want closure. That can be a beautiful impulse, but in excess it sucks the air out of the room and leaves no space for new ideas.
Second, it represents a process failure. Every society has a mechanism for generational transfer, renewal, some kind of changing of the guard. When the seats of power are filled with people over 75, it's evidence that this process has stopped working.
Finally, it predicts disruption. Someone who's 40, elected to an 8-year term in office, has a 97% chance of being able to finish. Someone who's 80 today, a coin flip. And those numbers don't count all the other risks, short of dying in office. Reagan had Alzheimer's towards the end of his presidency. When those things happen, real decision-making shifts from formal leaders to other, less accountable people around them. The shift can be gradual and subtle. History shows it to be dangerous.
We're in a difficult moment in America. I collected data about the top positions in our politics over the last 100 years, and it supports what I suspected. Our current political leaders are the oldest we've ever had.
Our president, the house speaker and senate majority leader were all born in the 1940s. Their median age is now 78.
I collected the Senate Majority Leader, House Speaker and President every year back to 1920. I could've gone further, but this looks like a fair proxy for the age of our senior leadership overall. The trend holds across different parts of society. The average newly hired Fortune 500 CEO was ~45 in 2000 and almost 60 now.
A single cohort, boomers, held much of the power then and still do today. As large voting block, they seem to elect people who are even older than themselves. A 78yo is a member of the Silent Generation, growing up before movies had sound.
Regardless of what happens this year, we are on track to break that record.
So the question is, what comes next?
 Using an actuarial table from the Social Security Administration
 Murman, The Impact of Age on Cognition.
 Salthouse, Selective review of cognitive aging.
 If Biden wins and the senate flips, it'll likely be Pelosi (80) Reid (80) Biden (78) for a median of 80.