I think it's not quite the case that people get more conservative as they age. It's that policies and goals throughout society tend to get more progressive with time – as we learn more about our needs and those of others – while personal ideologies tend to crystallize with age. When one generation solves a problem, the next generation starts looking for new problems to tackle. The trouble is that we have to do that while dragging the previous generations, kicking and screaming, towards something more broadly beneficial to society because they think they've already got it all figured out. The evolution of our understanding of the world doesn't end with our generation, no matter how much effort we put in. 🤷
Kids have turned out to vote Democrat and the promised change did not manifest. Remaining in status quo without progress does not motivate them to vote.