The average person's intelligence quotient, or IQ, is by definition 100. The average will always be 100, which means that a person's IQ may change through time depending on the intelligence of people that are born or die.
No IQ test is perfect, but can be useful to identify those with exceptionally high or low intelligence. People cannot study for IQ tests as it consists of questions that do not depend on the quantity of knowledge possessed by the test-taking individual. Because of that, having a low or high IQ does not indicate if a person is particularly knowledgeable, and ignores valuable traits like creativity and determination.