This gives me hope. I'm not even half his age, but already worry about my brain going. When I was 17 I could do basic calculus in my head. Today I struggle to carry numbers in long addition problems. It already has me thinking about my career and future - it certainly is downhill from here. Then something like this happens and just makes me feel like a big baby. Hats off Mr Steiner.
Don’t assume this is age. It could be a health condition beginning to manifest. Inflammation, vitamin deficiency leading to something bad, etc. Brain fog is a common symptom of many possible issues that may just be taking root. I would see a doctor and make sure I fixed any such issue I could.
I'm in my mid-20's and I can't do mental math as well as I used to. Why? I don't spend as much time on it now, as I did when I was in school some years ago.