This graph comes from data from the Woodcock-Johnson IV Technical Manual, p. 279-280. (large pdf, link opens in new window.). 

The Woodcock-Johnson is one of the top IQ tests, which uses a Rasch measure of intelligence, called a W-score. (The Stanford-Binet uses the same scale, but calls it “change-sensitive scale”, CSS). Rasch measures are absolute measures of ability, in the same way that lengths are absolute measures of distance – they have a true zero, so one can say this question is 10% harder than that, or equivalently, that one person is 10% more intelligent. The difficulties of questions and the abilities of test-takers are computed all together in a matrix with rows being test-takers and columns being questions, with each matrix entry being 1 if that person got that question right, and 0 if they got it wrong. Using matrix math, difficulties and abilities are computed simultaneously. A bootstrapping procedure can update the matrix with additional questions and test results, giving an ongoing update to test norming. After validating questions (a complex but not arbitrary process), the only free choice in making a Rasch measure is choosing a reference score that sets the scale, which for the W-score and CSS was chosen to make the average, 100 IQ 10.0 year-old’s score 500. 

I’ve made a graph of full test-scale (FSIQ) W-scores vs. age with additional lines for +/- 1 to 3 standard deviations. This allows comparing the absolute intelligence of people with different ages. So a 145 IQ (+3 s.d.) 8-year old can be seen to be best placed with a 130 IQ class of 10-year-olds, or a 115 IQ class of 13-year-olds. 

I’ve found a highly accurate curve fit for W-scores ages 5 to 17: 

 

W-score = 547- 473/age 

 

 Above 17, the average CSS rises to 520 by age 22 and remains 520-521 through at least age 35. Generally the W-score standard deviation in adulthood is 10.5. In childhood it falls from about 12 ages 5-8, to 9.5 ages 9-17. Assuming an s.d. of 10 is good enough for most calculations, since the s.d. data is quite noisy.

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Update June 5, 2021: 

Here is an improved version:

And here is a one-page PDF of it which can be scaled without loss of resolution: Absolute intelligence age distribution chart PDF