I’m 41 and feeling the same. One solution that helps me is to be more methodical, and make detailed notes. So, for example, when trying to understand a set of nested calls, I decide whether I’m going to do a “depth first” or “breadth first” analysis and then I work my way through it, noting as I go. And if it’s BFS, I do actually keep a noted queue of nodes to traverse. Then I can work mechanically without worrying about my limited memory capacity.

My memory is totally shot. I think it faded away through years of insomnia, poor diet and lack of exercise. It became harder to live healthily after I had kids.