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 29.5% under the age of 18, 6.7% from 18 to 24, 33.6% from 25 to 44, 20.6% from 45 to 64, and 9.6% who are 65 years of age or ...