> Is it reasonable to assume that the natural decline in cognitive performance over time is offset by the gains in experience and expertise?

Maybe up to a point. Most of the tools and languages I use daily are fairly recent, or at least new to me. I don't have much of an advantage, if any, compared to my younger colleagues.

There are certainly things I do better now than 10 years ago but I think I'm slowly declining though. Fortunately, there's more than one way to be productive professionally so I hope I can keep up for a few more year.