A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... the average family size is 2.79. In the town the population is spread out with 18.5% under the age of 18, 4.4% from 18 to 24, 20.6% from 25 to 44, 26.3% from 45 to 64, and ...