Age distribution fractions are estimated proportions of students in each grade that are age-eligible. The fractions are components in the school measure of size. The computation of the age distribution fractions was done differently by age group. For ages 9 and 13, a breakout by every birth-year cohort represented in the relevant grades was fully carried out. The approach was simpler for the age 17 sample, using an age range between 16- and 17-years-old, because this age definition is based on an October-September birth cohort.