I'm 46, and while I do think my cognitive speed has declined a tiny bit, my abstract thinking has improved throughout life. The one change I have noticed though is that programming has become insanely complex during the 37 years I have been programming, but more especially during the last 10-15 years. It's at the point now where you can't even create a basic website, to modern standards, without spending months of work to develop it, or without spending 95% of your time on StackOverflow wrestling with bugs and complexities of 7 different pieces of framework that you're trying to get working together. It's utter insanity.
What I suggest for your current situation is to diagram everything. Just buy some big sheets of butcher paper, and draw out the entire nested API structure. Rely on your visual sense so that you don't have to do as many mental gymnastics. Your visual processing center is the GPU of your brain, it has vastly more parallel processing power than the language processing centers of your brain.
Also remember that everyone has different aptitudes. You might not be decreasing cognitively at all, it might just be that this problem doesn't click with you like it does with the other developers.
What I suggest for your current situation is to diagram everything. Just buy some big sheets of butcher paper, and draw out the entire nested API structure. Rely on your visual sense so that you don't have to do as many mental gymnastics. Your visual processing center is the GPU of your brain, it has vastly more parallel processing power than the language processing centers of your brain.
Also remember that everyone has different aptitudes. You might not be decreasing cognitively at all, it might just be that this problem doesn't click with you like it does with the other developers.