> Is it reasonable to assume that the natural decline in cognitive performance over time is offset by the gains in experience and expertise?
It depends on the task, but overall, for the work I do as a software developer, yes.
I would say I have less energy, but I need less energy, and I produce better results in the end. I'm better at anticipating where a line of work will go, and I'm quicker and better at adjusting course. There are a lot of multi-hour and multi-day mistakes that I made ten and twenty years ago that I don't make now.
The raw mental energy I had when I was younger allowed me to write things I couldn't write now, but everything I write now is something that other people can read and maintain, unlike twenty years ago. It's very rare that writing a large, clever, intricate mass of code is the right answer to anything. That used to frustrate me, because I was good at it. I used to fantasize about situations where other people would notice and appreciate my ability to do it. Now I'm glad it's not important, because my ability to do it has noticeably declined. In the rare cases where it's needed, there are always people around who can do it.
Another thing that is probably not normal, but not rare either, is that the energy I had when I was young supercharged my anxiety and caused me to avoid a lot of things that would have led to better outcomes, like talking to other people. I'm still not great (as in, not even average for an average human, maybe average for a software developer) but I'm a lot better than I used to be.