I don't think there is an inevitable general decline of cognitive abilities with age, or the effect isn't strong enough to matter. At least, I haven't really seen them.

What I have seen is older people who are just tired of the constant personal improvement that any career needs to continue to flourish. People often find their niche and just want to stay there. That's a different thing entirely, though.

I'm a very senior IC in a FAANG and almost my whole job is going around and sharing 30+ years of wisdom with people on everything from "dealing with people" to architecture to pull request reviews.

Also, while I'm at FAANG now, I'm there because I was previously at a startup that was acquired by said FAANG, and if anything, there is even MORE of a need for this sort of wisdom intelligence in a startup. :)

This is a real issue at say 85, not at 45. You almost certainly aren't quite as quick as you were when you were 25, but on the other hand you typically know a lot more and the experience probably means you actually make far fewer errors overall...