It doesn't make a heck of a lot of sense with percentile-based IQs, but it does if you remember that IQs used to be ratio-based. So, someone with an IQ of 130 would have a mental age 30% more than his or her chronological age, and a person with an IQ of 70 would have a mental age 30% less than his or her chronological age. In terms of mental age, there would be a similar difference. A ten year old with a 130 IQ would have a mental age of thirteen, and a ten year old with an IQ of 70 would have a mental age of seven. They're both 3 years away from their chronological age and would, theoretically, need a similar degree of differentiation.
Now, how well the old ratio IQs correlate to the current percentile IQs (or how stable they might be over time) is anyone's guess. I couldn't begin to speculate.
