I suspect it's even simpler than that. If you simply assume that people don't change at all once they grow up, everything can be explained.
People are generally more selfish than we think and people lie more to fit in than we think.
Selfish people like social programs when they are young and might theoretically need help, but don't care about taxes because they earn little.
And they dislike taxes when they are old and have salaries and resources, but don't care about social programs because they don't qualify.
They don't care about politics or political parties because they really only care about themselves. They never really cared.
Others pretend to be left leaning to match their peers when they're young and right leaning to match when they're older. They don't care about politics as much as fitting in.