Yeah, I'm about the same age and started noticing my ability to keep everything in memory falling at around the mid 20s. I mean, I'm still probably way better at keeping all manner of obscure details in memory compared to the average person (we exercise that so much in this profession it's only normal), but it's below the peak point, not enormously so but it's kind like having once been a top "athlete", years later you know you can't reach that peak performance anymore.

Also once you go through the full life-cycle of enough of your projects (that got shipped and a year or two later you have to pick it up and change it), you kinda figure out that even at peak "performance" you wouldn't be remembering much from a project from years ago and start adding comments to help you pick it back up and alert you to possible pitfalls you noticed and avoided but forgot all about in the meanwhile.

Fortunally I figured it out long ago that I've created a couple of principles around commenting that have repeatedly saved my ass years later: things like documention parameter assumptions in functions, actually writting down the "why we do this" or commenting before the code of especially complex algorithms (I actually design the algorithm to the comment first and only after than code it).