A population's average IQ is based on an IQ figure of 100. As one grows, the increase in IQ tends to level off around 16 years of age, and thereafter increases at a much slower rate up to around the age of 20 years.
So the average teenager's IQ should be getting pretty close to the population's average figure of 100.
Lots about it here
Intelligence is distruted in the population by the 'Bell
Curve' which can be seem in the attached diagram. Strangely enough the Japanese are also distributed by the bell curve but is shifted to the right by 20 points. ie instead of the mean being 100 it is nearer 120.