Well, it is defined as the "mental age" divided by the actual age and multiplied by 100. The mental age is defined by the average test scores of people with that actual age. So, per definition the average mental age of an age group is exactly the same as the actual age of that age group.
That being said, the scales aren't really updated constantly so sometimes the test scores are compared to the average of a few years ago and apparently the average intelligence is increasing ever so slightly leading to the inaccuracy of an average iq slightly over 100.