This index method groups together Kanji that share strokes between them (particular the left-hand side stokes), for example the Kanji 試 and 計 would be grouped together.
... is spread out with 29.3% under the age of 18, 9.6% from 18 to 24, 30.3% from 25 to 44, 20.9% from 45 to 64, and 9.8% who are 65 years of age or older. The median age is ...