Warren Buffet at 93 is not operating with the mental clarity of Warren Buffet when he took control of Berkshire Hathaway at 32
Even if the stats predict he is not, stats are only a prediction and it cannot be assumed to be true for everyone. So we cannot say this with 100% certainty he is not as sharp now as he was at 30.
Here is one way to test this: compare the written works of aged authors compared to earlier works. how much of a decline is there in style, word choice, plot , etc.?