Memes

Young people tend to be more persuadable before 30, and tend to bake in their political views around that age. So big events in one's 20's tend to lead to lasting partisan affiliations for life after that.

FDR's presidency won over a lot of people to the Democrats in the 30's and 40's. Eisenhower's presidency shifted people over to Republicans in the 50's. Nixon pushed people away from Republicans. But by the 70's Democrats were losing a lot of voters, and then Reagan won a bunch of people over to the GOP. Then 9/11 won people over to Republicans, while the Iraq war pushed them away.

But each of these things had an outsized effect on those under 30. So Boomers who remember getting fed up with Democrats in the 70s and crossing over for Reagan (and then voting Republican in every election since) just thought it was the effect of age, rather than the effect of that particular political moment in 1980.

And even though this data and the analysis is mainly for Americans, it's probably reflective of how people shape their own political beliefs everywhere.