Old age begins at 27 - scientists claim after new research | Mail Online: "The research at the University of Virginia, reported in the academic journal Neurobiology Of Aging, found that in nine out of 12 tests the average age at which the top performance was achieved was 22.

The first age at which performance was significantly lower than the peak scores was 27 – for three tests of reasoning, speed of thought and spatial visualisation. Memory was shown to decline from the average age of 37. In the other tests, poorer results were shown by the age of 42."
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Actually, I think people get smarter as they get older. I was an idiot when I was 22, although I admit my reflexes were a lot faster. Some would say not much has changed except for the reflexes, but I beg to differ.