In my experience, it's been the complete opposite. The very experienced engineers that are actually willing to use top of the line tooling are much better than they were before, including those that are over 40, and over 50. Part of the practical degradation of traditional programmers over time has always been concentration and deep calculation, just like in chess. The old chess player knows chess much better than a…
I don't know if I agree that this is the bottleneck.
What I can agree on is that as I have aged I now simply REFUSE to learn programming knowledge that has a half-life.
Phone programming? Nope. Front-end web programming framework? Oh, hell, no. Build system of the month? Piss right off. etc.
AI lets me fill in that kind of programming with "acceptable" (read: super crappy but I didn't have to think about it) results because that code won't exist in 5 years anyway due to its half-life.