For a long time I looked around at the relative lack of 40+ developers and founders in my field and considered it worrisome; what will I do when I get to that age? do I need a massive bank of savings just in case?
But I think it's also easy to see it as an opportunity. We are staggeringly lucky generation; software is the future and it isn't going the way of mass automation or obsolescence any time soon. There is probably another two hundred years of foundational innovation left in the field, we're still just scratching the surface of information theory and what's possible. Experience is likely to increase in value.
As individuals; by cultivating just a little higher order thinking, and seeing above the surface layer of raw code, it shouldn't be hard to offer meaningful value and insight for the remainder of one's lifetime. I don't think the key is learning to "adapt", progressing from one fad language or technique to another, so much as it is to see the similarities between these fads and to understand them at a higher level of abstraction. To understand that SOA programming is the same thing as micro-kernels, or CSP, for example, and to seem to offer new wisdom by regurgitating the old.
In that spirit, I've observed that the successful senior engineers I've come to know don't decry constant change, but constant same-ness.