A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... the city the population is spread out with 27.2% under the age of 18, 8.7% from 18 to 24, 29.5% from 25 to 44, 21.0% from 45 to 64, and 13.5% who are 65 years of age or ...