2000 18,261 1990 18,129 1980 19,604 1970 20,352 1960 20,393 1950 19,799 1940 18,927 1930 19,412 1920 19,241 1910 19,349 1900 20,148 1890 19,034 1880 19,238 1870 17,626 1860 16,193 1850 16,445 1840 16,456 1830 9,707
As of the census of 2000, there were 18,261 people, 6,923 households, and 5,046 families residing in the county. The population density was 45 people per square mile (17/km²). There were 7,337 housing units at an average density of 18 per square mile (7/km²). The racial makeup of the county was 97.69% White, 0.60% Black or African American, 0.16% Native American, 0.47% Asian, 0.02% Pacific Islander, 0.25% from other races, and 0.82% from two or more races. 0.50% of the population were Hispanic or Latino of any race. 32.5% were of American, 27.6% German, 12.7% English and 10.3% Irish ancestry according to Census 2000.
There were 6,923 households out of which 34.30% had children under the age of 18 living with them, 60.80% were married couples living together, 8.40% had a female householder with no husband present, and 27.10% were non-families. 23.30% of all households were made up of individuals and 11.30% had someone living alone who was 65 years of age or older. The average household size was 2.60 and the average family size was 3.06.
In the county the population was spread out with 26.70% under the age of 18, 7.50% from 18 to 24, 28.90% from 25 to 44, 22.20% from 45 to 64, and 14.80% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 96.60 males. For every 100 females age 18 and over, there were 93.20 males.
The median income for a household in the county was $38,152, and the median income for a family was $42,633. Males had a median income of $32,491 versus $22,101 for females. The per capita income for the county was $17,997. About 5.50% of families and 7.30% of the population were below the poverty line, including 7.00% of those under age 18 and 11.40% of those age 65 or over.