Technically speaking, it's true the slope must by necessity decrease as you get older, but the effect is grotesquely oversold. It can be compensated for by the fact that when an older person goes to pick up some new tech, they're quite likely to have some previous tech like it they've already picked up and just be filling in some gaps instead of starting from scratch.

I don't think the correlation is large enough in practice to be a useful metric. I'm pretty sure I learn new things faster than I did twenty years ago because usually I have a lot less stuff to learn. Learning how to spell queries for your tenth database is much easier than the first one, for instance, and I've got SQL, NoSQL, columnar, embedded, and I dunno how many other adjectives of experience vs. some relatively young programmer encountering their second database type. My slope on such a task can approach vertical whereas a younger person may need a couple of weeks.

(This example is on my mind because literally today I'm helping someone diagnose why a query is slow on a database I've never even used before. But I understand the fundamentals of how the DB works, and can read the docs really quickly now, because I don't even know how many DBs I've used at this point. Depends on how you define them.)