Cognitive development and average IQ
A person's cognitive development is tied to his/her age. As a child grows older, it gets smarter every year. At the end of adolescence the cognitive development stabilizes. An 18 -year old will get only slightly better results for an IQ test than a 16 year old. If people stay healthy, their intelligence will remain stable at least until after they turn 65.
Around the 75th year the decline sets in: people will then start scoring lower on IQ tests. This means that adults of all ages can be compared reasonably well.
The deviation IQ
Simon and Binet, the inventors of the first intelligence tests, calculated the IQ as (mental age/chronological age) X 100. Obviously, this works well if you are dealing with children. Adults however, do not get smarter every year, so this is not the right way to calculate the IQ of adults. That is why the switch has been made, for both adults and children, to the so-called deviation IQ.
The deviation IQ compares a person's IQ test score to the average IQ of a person of that age. The average IQ is equal to 100. If your score is higher than 100, you are smarter than the average person of your age; if your score is lower, you are (slightly) less smart than the average person of your age.
This way, the degree of cognitive development can be measured and expressed in an IQ number. The average IQ of your age group therefore always is 100.