A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... from 18 to 24, 33.8% from 25 to 44, 21.3% from 45 to 64, and 11.8% who are 65 years of age or older. The median age is 36 years. For every 100 females there are 96.3 ...