I don't think that because Boomers and (a lot of ?)GenX tended to grow more conservative as they got older proves it's 'the natural order'.. Millennials are approaching 40 now and the liberal ones I know are still very liberal..
I'd argue that Truman getting elected in 1948 meant that there were a lot 'progressive' older folks back then. Especially since there was a more conservative "Dixiecrat" on the ballot as well.
So growing old and becoming conservative may not necessarily go hand in hand...