As you pointed out, we all have our biases. If I were raised racist, I'd try to confirm those biases in my personal experiences. Or perhaps some kind of personal experience would shatter my biases. I could be on the lookout for things I hear on talk radio, or I could make unexpected friends. Equally, I could get my sense of morals from reading Ayn Rand, Nietzsche, St. Augustine, or the Dhammapada. I've known people who have learned atrocious or wonderful things from books, and I've known people who have invented fantastic or horrifying personal philosophies. I don't think I've uncovered any kind of pattern as to why some people think one way and others another.
The funny thing here is that in this discussion, we're using our personal experiences (anecdotes and armchair speculation) to try and analyze it.
My personal contribution is that older people have more property, and that whatever material they use for their political views (be it books or experiences) will mostly be used to support their own goals for enjoying that property. The young folks want to get property, so they vote for that instead. That doesn't explain much, but it does partly explain why younger citizens tend to be more liberal.