I went from junior dev / 'young hacker' type profile, to a VP and exec level management of SMB class orgs, also did Silicon Valley roles in both eng and management.
My take, by far experience makes you able to solve problems in a way that is basically not possible taking a 'young hacker' approach. I can't say anything about cognitive decline other than forgetting my car keys - when it comes to tech the muscle memory seems to vastly overcome that plus some.
I think what we mistake for age decline often - is time allocation related. I know younger devs working and sleeping at office (and loving it). Where I can't do this due to many factors - kids, family, hobbies, overall a larger desire for non work related time. This explains the majority of 'knowledge gap' I see at work. Domain knowledge, especially if the tech debt variety only gets one so far however. In my experience leveraging experience and judgement far outweighs knowing a flavor of month curly brace language syntax.
The single biggest detriment to techie career progress I see is 'writing the wrong software'. Startups that go nowhere. Projects with no value. Solutions with no problem to solve. The 'slightly pessimistic' bias experience gives when writing software seems to catapult one -way- beyond what's possible otherwise. In fact this same bias I think is what enables engineers more experienced to drive up productivity of entire teams/orgs/corporations even.
This reminds me of the old joke about a very expensive consultant after fixing machine with one hit of a hammer. "My price includes 1% cost to swinging hammer, 99% to knowing where to hit."