A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... size is 3.47. In the town the population is spread out with 28.2% under the age of 18, 7.5% from 18 to 24, 33.5% from 25 to 44, 23.9% from 45 to 64, and 6.9% who are 65 ...