This is NOT normal and should not be accepted as such.
I'm 63 and have been programming professionally for 40 years. I'm currently working on several projects that are as complex as anything I've ever done. I honestly feel that that I get better every year, build stuff I never imagined a few years ago. I know this is subjective, but I feel like my brain is faster, I have more stamina, and better memory than ever. I still do newspaper jumbles and crossword puzzles without a pencil and plan to continue that way indefinitely.
Unless you're managing a serious medical condition, I suggest you do something differently: eating, exercise, lifestyle, medical care, something.
To you kids out there (under 50), do not despair. The best could be yet to come if you make it that way.