>There is some (unintentional I'm sure) age bias even in the response of "stay relevant" as if the implication is that older devs don't do that as a matter of course while younger ones do.

Take two doctors. Both learned all their knowledge going through med school. Neither did anything else to stay relevant. One graduated 5 years ago, the other 25 years. Which is likely to have more relevant knowledge? Is it because they stayed relevant? No. It is because everyone has some learning period (be it through college or not), and on average older workers had that period longer ago than younger workers, so the relevancy bonus has decreased more.

The longer since you had a significant learning period (the longer since you graduated school), the more you have to work to stay relevant. This isn't tied to age, only correlated because it is rarer for a younger person to have had their learning period (graduated) longer ago.