I saw this in two aspects.

I first noticed this when I was in my 30's taking advanced technical courses with smart people younger than me. My thoughts were slower than theirs and it was harder for me to understand the concepts.

During my early 50's I slowly and unknowingly developed a "brain fog". This was caused by very high blood sugar. My eyesight was affected so it seemed to be a literal fog. After I got educated on carbohydrates and eliminated them, the symptoms went away.