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.