As a casual reader of the thread, with little or no related knowledge:

The years add depth and complexity to our filters for processing new knowledge. Is the delay measurable? Probably. Is that 'decline'? Maybe it looks like it by the simplest standards.

Is there a meaningful analogy in neuroscience where simpler and more robust 'structures' - superficially appearing as a degeneration - could be a reflection of efficiency and optimization?

I can tackle complexities in my 40s that I couldn't dream of in my 20s. (Maybe I'm a late bloomer.) If my 40s -> 60s is anything like my 20s -> 40s, mentally, I'm just a little bit excited to be honest.

Total outsider thinking out loud. The suggestion that our 20's is any sort of peak (besides maybe animal / physical) gives me a laugh. I was a simpleton in my 20s.

From the neuroscience side not that I'm aware of. Getting old is basically the process of your body not being able to repair itself anyway.

I think the apparent conflict resolves if you consider that you have learned to be a better problem solver, a better learner, such that a small decrease in some 'raw ability' is completely overshadowed by gains from learning.

For a concrete example I'm much better at close reading now. Even if younger me was 10x as intelligent there are problems he'd bounce off that I wouldn't simple because I now know how to slow down and read thoughtfully.