A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... is 2.43 and the average family size is 3.04. In 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% ...