I'm not sure it was ever accurate for people who weren't already conservative.

It makes a lot more sense that, as you get older, you stop growing and learning, so as society progresses, your formerly progressive views become commonplace and eventually anachronistic.

(That's 100% what happened to my mother, who was a hippie, literally flowers in her hair, and now "just doesn't really get the whole trans thing")

And, if a person was progressive, but had some secret conservative or regressive values, those values come into sharper relief when their other views become commonplace -- and, as you get older, you're less interested in hiding your flaws and/or shameful values, so they come out more.

(That's what happened with my dad, he was in folk music groups in the 70s and then became a doctor and didn't like the idea of poor people getting some of his money (even though it was those same programs that kept his mother afloat after his father didn't come back from Korea).)