If you take on of the ready-made tests of intelligence and try it on a very
large number of persons, you will find that they succeed with it in very
different degrees. Repeat the test, and you will find that they cannon, with
the best will in the world to do well, alter their scores very greatly. Then
give the same group another test, and you will discover that the difference
among individuals are approximately, although not exactly, the same. And you
can go on. You will find that an adult, after continued exposure to his
social and educational environment, does not greatly alter his score on a
given test; that children, however, do steadily improve their performances
until somewhere between ten and twenty years old; that the average age at
which improvement stops is about 14 years...