Correct me if I’m wrong but the saying comes from the responsibilities people have at different ages and not a hard and fast rule. When you’re young you’re likely more progressive and you’re not the one having to work and worrying about how far you can stretch a dollar. When your older you’re more conservative because you’re having to work and pay for all your expenses which at that point you want less government taxing you to pay for liberal shit.
Edit: Liberalism of the past and what’s today are different today’s Liberals are extreme progressives in disguise. So, I may have conflated the two.