"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 ?