A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... the population is spread out with 24.7% under the age of 18, 6.9% from 18 to 24, 36.3% from 25 to 44, 25.1% from 45 to 64, and 7.0% who are 65 years of age or older. Th ...