Then there seems to be something missing here. I agree -- academics is only one area of success. But if gifted does not correlate with academic success, what kind of success are they talking about? Marking certain kids as gifted is a useless exercise unless that information actually translates into some kind -- any kind -- of success. If there is no predictive power to the test, then, at best, the test is useless.
It depends on your definition of success. Most people would not define success in life by their GPA. This article is telling us that high-iq children are not academically successful (as measured by their school grades).
Even worse, they don't explain how they calculate mental age
"Mental age" can be synonymous with IQ. I assume thats how the author of the article was using the phrase. Some variations of IQ tests present IQ as a ratio of mental age over chronological age, especially useful for testing children who are expected to be underdeveloped compared to an adult.