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It compares people of the same age or age category and assumes that IQ is normally distributed, that the average (mean) is 100 and that the standard deviation is something like 15 (IQ tests sometimes differ in their standard deviations).
Hence, the normal or average IQ score is 100, which means that the mental and chronological ages coincide. It is true that modern intelligence tests now apply statistical methods to produce a score reflecting one's performance compared to the average performance of his peers.