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.
I think this is interesting so I tried to find information about the distribution of ratio I.Q. scores, but there wasn't much easily available on the internet. I saw one item that referenced them being "roughly" Gaussian.
But let's go with the ratio idea - take 3 hypothetical 6 year-olds with I.Q.s of 150, 100, and 50. One has a "mental" age of 9, one of 6, and one of 3. It seems to be that the 9 year-old and the 6 year-old are going to be closer in ability than the six year-old and the 3 year-old.
