2000 20,391 1990 19,490 1980 19,361 1970 17,134 1960 16,074 1950 16,174 1940 16,211 1930 16,713 1920 18,400 1910 20,676 1900 22,407 1890 22,060 1880 22,122 1870 17,998 1860 14,556 1850 8,616 1840 6,305 1830 3,196 1820 1,882
As of the census of 2000, there were 20,391 people, 7,569 households, and 5,752 families residing in the county. The population density was 51 people per square mile (20/km²). There were 8,333 housing units at an average density of 21 per square mile (8/km²). The racial makeup of the county was 97.70% White, 0.60% Black or African American, 0.22% Native American, 0.19% Asian, 0.01% Pacific Islander, 0.74% from other races, and 0.53% from two or more races. 1.49% of the population were Hispanic or Latino of any race. 40.6% were of German, 21.7% American, 8.9% English and 8.2% Irish ancestry according to Census 2000.
There were 7,569 households out of which 35.40% had children under the age of 18 living with them, 65.00% were married couples living together, 7.20% had a female householder with no husband present, and 24.00% were non-families. 20.80% of all households were made up of individuals and 10.00% had someone living alone who was 65 years of age or older. The average household size was 2.65 and the average family size was 3.07.
In the county the population was spread out with 26.50% under the age of 18, 7.30% from 18 to 24, 29.10% from 25 to 44, 24.10% from 45 to 64, and 13.00% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 100.40 males. For every 100 females age 18 and over, there were 100.00 males.
The median income for a household in the county was $42,451, and the median income for a family was $49,123. Males had a median income of $35,125 versus $22,787 for females. The per capita income for the county was $18,000. About 4.80% of families and 6.90% of the population were below the poverty line, including 7.90% of those under age 18 and 7.80% of those age 65 or over.