A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... the town the population is spread out with 24.7% under the age of 18, 7.4% from 18 to 24, 32.6% from 25 to 44, 23.2% from 45 to 64, and 12.2% who are 65 years of age or ...