A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... age of 18, 5.0% from 18 to 24, 29.0% from 25 to 44, 24.2% from 45 to 64, and 12.7% who are 65 years of age or older. The median age is 39 years. For every 100 females ...