What exactly? The idea posed in the OP or the test?
Both, but more so the test. The association of mental age with IQ testing was unwarranted from the start. The original tests assessed a person's knowledge of the American culture more than anything else. Robert Yerkes was notorious for creating tests containing such howlers.
Today, the examinations have improved to emphasize problem solving and abstract reasoning more, yet the exercises they offer are still far too simplistic to measure a person's true intellectual abilities or even less potential. Strikingly, the official IQ test still contains questions of basic mathematics and facts regarding politics and culture.
That is so mostly because a person of merely average intelligence can too easily learn to solve those problems by route just as he can attain a BA or an MA whilst having learned little and only slightly improved his skills in scholarship.
In order to measure a person's true intelligence, we'd need a test that forces a person to solve recondite, theoretical problems rather than puzzles the range of difficulty of which does not exceed that of common-place crossword challenges. Obviously, Albert Einstein is to be regarded as a genius because of his superb abilities to understand and create abstract ideas and not solving simple puzzles with a lightning speed.
Today, the view that IQ tests could be manipulated by their test-takers is quite common among practicing psychologists and has become prominent as one of many criticims of the Bell Curve authored by Herrnstein and Murray.