When I was 40, I think my cognitive capacity and coding ability was pretty good - possibly peak. By the time I was 60, I found learning new stuff (especially languages and frameworks) hard; but that was partly or wholly because I didn't regard learning new tricks at that age to be a good investment of my time and effort, since I'd soon be retiring.
Since I retired, I have suffered a cognitive decline; everything takes longer. But, you know, I'm retired, and work expands to fill the time available.
I'm 53, I'm feeling similar. I see the 7th package manger or the 4th pipeline manager or the 17th linux disto or the 52nd video game where you can craft shit...
and it just makes me tired. While I'd like to retire and think I'll financially get there sooner or later, I'm afraid that there's an underlying structure to work that I'll miss.
Since I retired, I have suffered a cognitive decline; everything takes longer. But, you know, I'm retired, and work expands to fill the time available.