I think the primary factor is that the society itself becomes more progressive, so definitions of "progressive/conservative" change. Sixty years ago, segregated schools were supported by mainstream conservatives, and same-sex marriage would have been considered a radical leftist agenda. An "ordinary" young person from 1960s, growing old without changing their opinions, would seem hopelessly out of touch today.
I would have assumed that as well, but it seems a lot of 'social norms' I thought had changed, really haven't..from racism, to sexism, to hate crimes..It really seems we've evolved very little from 100 years ago.
