Originally Posted by Peter
Technically, IQ x chronological age is mental age. Average IQ of 100 at 5 years old starts at K level but with Gifted student with IQ of 140 already is 2nd grade level and IQ of 160 would be in 3rd grade level at 5 years of age.(Granted that the kids are being challenged/stimulated).

The research has shown that IQ correlate better with Math than reading level. Some kids with above grade reading level may have average IQ.
That may have been true of the old IQ tests, where IQ was calculated as a ratio of "mental age" to actual age, but these days IQ scores are based on how many standard deviations from the mean the scores are. And I think the norms are done for every age, but I'm sure we have people who know how testing really works who can explain better.