I just posted in the GT Research forum a link that is relevant to your question: Converting IQ at a Given Age to an Absoloute (Rasch) Measure of Intelligence

For instance, this allows telling how smart your 145IQ 7 year-old is compared to a 120IQ gifted 13 y.o. class (about the same).

Here's a more detailed description of how it works:
To compare the absolute intelligence level of two people of different ages and IQs, first convert the IQs to standard deviations, s.d. (= z-score = sigma) = (IQ - 100)/15

Then consult the graph at the link, find the age, move up a vertical grid line to the s.d., (e.g. if the IQ is equal to +2 s.d. then find where that vertical grid line intersects the second red line above the average (black line)), then use the horizonal grid lines to find the corresponding score on the scale on the left. Note that score. Do the same for the other person's IQ and age. Now you have two comparable scores.

You can compare the difference in their scores to the size of the standard deviation at the younger person's age (vertical distance between red or black lines at that age), which allows converting the older person's score back to IQ-equivalent for the younger person's age (so that they're comparable).

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