Most IQ tests are calibrated to the age of the person taking it. The scale gets adjusted or special tests with slightly different tasks are used for different age groups. The main idea is that within each age group the average IQ is 100 by default. Thus I think that a calculation of a "mental age" is only neccessary if your results have not been adjusted to your age, KissOfMarmaladeSky. If the person doing the test with you has taken your age into account than your IQ is 113 and thus a bit above the average of your age group.
As far as I know the reason fur adjusting the results according to the age of the person taking it is that the human mind developes further as the person matures and works a bit differently plus everyone simply gains more experience which the brain can put to use when working on an IQ test. It would simply not be fair if children, adults and seniors were all expected to perform equally well in an IQ test without age-adjusted scales, because the have different potentials for handling that test.
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