Of course our brains slow down as we age. My brain is much slower than the average 23 year old’s. But I’ll bet dollars to donuts that I’m still far smarter than that young whippersnapper.
You can see it in some respects like memory (mine is going fast), but now, at age 65, I can finally read Nietzsche, Sartre (barely), and Hegel, whereas before, I couldn’t make sense of them. So in some ways, I’m smarter than ever.
Brain speed is maximum at age 23, coincidentally when we have the most brain cells. This is fluid IQ, a measure of raw brain speed. But there’s not much in a 23 year old’s brain. So you have this fast brain zinging around looking for “knowledge blocks” and not hitting anything.
On the other hand, our older brains are much slower than the 23 year old’s, but as soon as the zinging starts, it hits one “knowledge block” after another after another, and furthermore, a lot of the knowledge it is hitting is linked or links up to other knowledge. This is crystallized IQ and it can keep rising in the 40’s, 50’s, 60’s, 70’s, and even 80’s! And the fact that our tremendous knowledge has linked up a lot with other bits of knowledge allows us to form and see patterns.
Otherwise known as “wisdom.”