I'm over 35 and my productivity and capacity to learn new things has only continued to grow over time.
It's true that many exceptional artifacts have been produced by people when they're relatively young. I think this is more a byproduct of ignorance (i.e., seeing the world with a fresh pair of eyes). But as this list shows, maybe age isn't the critical factor -- maybe it's about diving into something new and unfamiliar.
I'm still just as productive as I was in my 20's, but now I've got decades more experience than my peers and can see things that the hubris and arrogance of youth never let me see before - like, how important it is to be able to work with others, not hold onto projects selfishly .. how important it is to check oneself before one wrecks oneself, and so on.
If I had a way, there'd be so much advice I'd give to my younger self...
Maybe our industry just grinds the greatness out of you by 25 or 30 unless you actively fight it.
Part of that 20 years' experience, of course, is learning to ignore 'best practices' that actually aren't 'best' at all (or more likely, are 'best' for some situation you're not actually in).
Maybe people just get tired of churning out CRUD apps after 10 years and want to do something else. A lot of stuff loses its charm after you see behind the curtain.
I also remember someone bringing up a good point (it might have been here or something I watched, I can’t remember) that software engineers don’t actually get that many ‘attempts’ at doing a full software system from scratch. How often does the average software engineer get to design a system from requirements, build a system from scratch, take it through production and then into maintenance in that length of time?