Lots of things can happen to your brain as you age. But if it's physically healthy, there's not much difference between people of different ages. Noam Chomsky is 93 and, though he speaks more slowly, his mind is sharp as ever. It's just that, statistically speaking, older people have a higher chance of physical problems inside the brain or brain cells, probably many we don't even know about yet.
Same can be said of Donald Knuth. He even made a shorter proof than the proof that was considered to be a proof from the book years ago, moments after the original proof was published.
I mean, they all might be outliers for as long as they live and the general conclusion (of decline) applies to us regular folks.
And speaking more slowly could be the result of years of experience that taught him to think carefully about what he says, especially in non-casual settings.
