Ahh…the brain at retirement! But there’s another aspect to “losing the edge”, and it’s not all negative.
I’ve started to notice the same thing. I’m re-reading many of the books on my shelves, Some of them are still interesting, but others less so. And that’s the main issue, I think, in what you call “the edge I’ve lost” . It’s not so much the ability to concentrate, but it’s more how much you care about it.
I’m enjoying many of my old books. But I also have a few college textbooks–and ,well, let’s just say that at my age, I ain’t re-reading that shit. And one reason is that , being older, you know what in that book is important, and what was a waste of time.
Your example of the “6 principals of xxy CRAP”, is great. In college, we were all enthusiastic about everything,-- every word the prof said must be important, right? The prof says that there are 6 principals of xyz, so we paid attention, not just for the exam, but because , gee whiz, principals are important!
No 19 year old student will say “those principals are crap”. But now at retirement age, we know better.
In a formal college setting, there’s so much that is just theory and never has an practical use. Young people don’t know the difference , so they absorb it all. But at 70, I have less patience, and a whole lot less interest, and I am confident enough to know that it is crap.
So don’t feel bad about yourself for losing your edge, and your ability to concentrate, and recall stupid details. It’s not because you are old, it’s because now you know which details are stupid.