The term IQ, or Intelligence Quotient, generally describes a score on a test that rates the subject's cognitive ability as compared to the general population. IQ testing was formally named in 1912 by a psychologist named William Stern. The idea is that a person's average IQ score accurately represents their mental age in conjunction with their actual age.
Mensa gives the example that a 10-year-old with a mental age of 10, therefore, has an IQ score of 100. However, if the mental score is lower than the age, it drags the IQ score down. So, if the 10-year-old's mental age is 8, their score would only be 80. But, if their mental age was 12, their IQ test would reveal a score of 120. It's clear that modern IQ tests do take into account the age of a child when determining an IQ score, as children are graded relative to the population at their developmental level.