A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... town the population is spread out with 13.8% under the age of 18, 34.0% from 18 to 24, 17.6% from 25 to 44, 20.5% from 45 to 64, and 14.1% who are 65 years of ag ...