You’ve maybe heard someone say "I opened my mouth and my mother spoke out of it." Home attitudes and young peer attitudes are the big forges.
But I wonder how much uniformity arises from popular amusements? Everybody has a TV and almost everybody has cable, and the kids grow up acquiring at least some common residuum of shared values. I’m not arguing that the shared values are noble or useful, or have redeeming qualities; but they’re shared and should to some extent dilute the folkways they were born to. Do we see this? Can we distinguish folk-way trends between those who spent a lot of boob-tube time before they were 7 and those who didn't (within the same area, of course)?