A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... 3.11. In the village the population is spread out with 27.6% under the age of 18, 6.2% from 18 to 24, 17.7% from 25 to 44, 36.0% from 45 to 64, and 12.5% who are 65 years ...