Some recent research indicates that though there are some notable cognitive declines (primarily in memory and processing speed) following early adulthood, the impact of these declines are not all that significant.  We often harshly judge our our memory skills and do so in a manner that is simply not very accurate.  In fact, some studies are suggesting that despite these memory slips and cognitive slowing, our critical thinking, judgment and decision making improve so significantly with age (and experience) that it compensates for the cognitive losses.  An example given was a study that compared reaction times of young pilots to more senior pilots: the younger pilots responded more quickly in simulation practices, but the senior pilots crashed less often.