Martin writes:

How was the average mental age of the US population computed?

The average IQ is computed by administering a test to a large and
representative sample of the population on which the test will ultimately be
used, and then assigning 100 (an average IQ score, by definition) to the
average raw score obtained by the sample cohort on the test. The scoring is
then further normalized to ensure that a constant deviation in IQ score
represents a constant percentile. Usually 15 is the width of an SD in IQ
points, such that 130 represents two standard deviations above the mean in
scoring, or the 98th percentile.