By nature men are nearly alike. By practice they get to be far apart.
People are not all the same. There are young people, old people, and people in between. Nevertheless, we do make distinctions, ranging from binary decisions about the right to vote to the minimum age at which one qualifies for a pension. Many of these distinctions are based on the underlying presumption that intelligence grows, and then declines. There are mandatory retirement age requirements for air traffic controllers and commercial aviators, largely because of our beliefs about changes in their cognitive capabilities. In the United States federal judges are appointed for life. Many of the individual states have mandatory age limits for judges.
There are men and women, a sharper biological distinction than young and old. Historically, many societies have assumed that men and women have different cognitive capacities. In contemporary post-industrial societies there is a presumption of equality. But does “equality” mean “identity?” Should we regard a low incidence of women in corporate law practice with suspicion? Should the acceptable percentage of women in corporate law practice be the same as the acceptable percentage of women working as helicopter pilots?
People are identified with, and self-identify with, various racial and ethnic groups. Do these groups differ in intelligence? To some people the answer is obvious – they do. Other people believe that even the suggestion of a difference is evidence that the speaker is prejudiced.