"progressive people, and their opinions, turn into reactionary conservatives with nothing but the passage of time."

I'm sorry - that line makes it sound like I am saying those people who were progressive become, themselves, reactionary conservatives.

What I am trying to say is that viewed by successive, future generations those people become reactionary conservatives. That is, simply the passage of time changes the view, from outside, of those people.

"Many of today's conservatives are quite young, and lots of liberals are quite old."

Agreed. What I am saying is that, absent a revolution or other reset of our current society, both of those groups will be considered, 200 years from now, quite conservative.

An example looking backward:

Find me the most liberal and the most conservative participants at the signing of the US Declaration of Independence - now let's sign them up to speak at Berkeley tomorrow and see how that goes. See what I'm saying ?

You're falling victim to the classic fallacy that societies get more liberal with time. That simply isn't true at all, and you can see it many times in history. Here's a couple of examples: 1) Roman society vs. Medieval feudal society vs western society in the 15-1800s. The Romans had slaves, but the slaves could own money, buy their own freedom, etc., so they were really more like "indentured servants", so this already makes them more liberal and respecting of human rights than America in the early 1800s. Romans had freedom of religion: no one was burned at the stake for believing the wrong kind of religion there. You can't say the same for Medieval times. 2) Ancient Greece vs. everything until about 40 years ago: in ancient Greece and nearby areas, homosexuality was not only tolerated, but encouraged among military members to improve unit cohesion. We're only now tolerating homosexuality again, and even then, not that well. 3) America in the 1920s vs. America in the 1940s-60s. Remember "flappers", speakeasies, etc.? Society was more liberal in the 20s than later.

I can easily find examples of people in my own social circles (mainly through family contacts) where there's young people who are die-hard conservatives, MAGA fans, Christian zealots, etc., and older and elderly people who are staunch defenders of Roe v. Wade.

The things you identify as "liberal" vs. "conservative" simply do not correlate very well with age or generation at all.