About.com does not cite age as a factor in deciding someone's IQ. Therefore, the average IQ of a 15-year-old would be about 100, which is the average for every other age.
About.com states that IQ scores are calculated by comparing the results of each individual to what is considered to be the norm. Intelligence quotient scores fall under the bell-shaped curve pattern. This means the majority of the participants fall towards the middle of the spectrum and that there are less examples the farther the distance from the middle in either direction. For example, it is just as likely for someone to have 75 IQ as it is for them to have 125 as both of these values are equidistant from the average value of 100. The same would go for any two other points that are equidistant from the middle point.