the prefrontal cortex is susceptible to age-related decreases in blood flow, changes in the structure of cells, and reduction of volume (shrinkage). A biological explanation is that there is no evolutionary pressure for the prefrontal cortex to stay sharp in old age, just as there is no evolutionary pressure for anything to stay sharp in old age. (Once we are past the age of reproducing and passing our genes to the next generation, evolution doesn’t care about how we spend the rest of our lives.) And so processing speed, led by prefrontal cortex decline, slows down.
