An intelligence test (IQ test) is, by definition, any test that purports to measure intelligence. Generally such tests consist of a graded series of tasks, each of which has been standardized using a large, representative population of individuals. This procedure establishes the average IQ as 100.
It is generally believed that a person’s IQ rating is hereditary and that the rate of development of a person’s mental age remains constant until about the age of 13 years, after which it slows up. Beyond the age of 18 little or no improvement is found.