It's a damn good thing:
1) there is a tremendous redundancy built into the human brain, so we can lose functioning neurons and still not lose significant mental function
2) the benefits of experience and learning greatly outweigh the loss of computing speed, so the brain can continue quality reasoning well into the sixth, seventh, and eighth decade or beyond.
3) good, solid reasoning and quality creativity are not races that go to the swift.
4) there are some aspects of brain fuction (as well as other aspects of human biology) where quick is not synonymous with better.
5) barring catastrophic or disease-related events (including dementias), learning is a life-long human quality
6) professional athletes rarely compete at top levels into their 40s, but intellectual athletes can continue to "compete" at top levels several decades beyond that.
Sorry, old farts (this includes me). Excuses are still just that--excuses. Keep the mental whips cracking. It's up to you as to whether you want to continue to compete as an intellectual athlete or not. Your brain can take it. Can you?