An interesting observation from an inside perspective. I was aware of a few physicists who had "peaked" early in their careers, Einstein was a prominent example. That poets show the same pattern is something I just learned. About painters and other visual artists, it's true that many were still working and producing in their final years. OTOH it was often the case that in old age they worked in relative obscurity as…
When you're 20-30 something and a nobody in your field, but a privileged mind, you keep your eyes open to all posibilities. You are hardworking and then results come.
Then you gain a name and have all eyes on you and your mental processes change. Suddenly with your opinions, you've got a reputation to defend. You're aware of those supporting you and those who are not. You're reasoning gets blurred by your ego and personality. Once you publicly speak an opinion on a topic for enough time, you cannot simply go back without looking like a moron.
There are several instances of the most brilliant minds following this pattern. Geniuses giving their backs to the scientific method, to all evidence, and looking like amateurs.
Comes to mind Einstein, spending his last years trying to debunk quantum physics against all evidence, Linus Pauling and his C-Vitamin craze or Nikola Tesla and his every time crazier projects with no practical viability. I guess there might be more.