A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... is spread out with 26.0% under the age of 18, 9.4% from 18 to 24, 34.6% from 25 to 44, 19.4% from 45 to 64, and 10.6% who are 65 years of age or older. The median age ...