As of the census of 2000, there were 19,922 people, 7,574 households, and 5,476 families residing in the county. The population density was 48 people per square mile (18/km²). There were 8,005 housing units at an average density of 19 per square mile (7/km²). The racial makeup of the county was 97.25% White, 0.30% Black or African American, 0.16% Native American, 0.26% Asian, 0.01% Pacific Islander, 1.32% from other races, and 0.70% from two or more races. 3.46% of the population were Hispanic or Latino of any race. 30.9% were of German, 21.1% American, 12.4% English and 9.1% Irish ancestry according to Census 2000. 86.8% spoke English, 7.6% German, 3.6% Spanish and 1.6% Pennsylvania Dutch as their first language.
There were 7,574 households out of which 33.50% had children under the age of 18 living with them, 61.10% were married couples living together, 7.90% had a female householder with no husband present, and 27.70% were non-families. 24.70% of all households were made up of individuals and 12.30% had someone living alone who was 65 years of age or older. The average household size was 2.59 and the average family size was 3.10.
In the county the population was spread out with 27.00% under the age of 18, 8.00% from 18 to 24, 26.80% from 25 to 44, 22.10% from 45 to 64, and 16.00% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 94.40 males. For every 100 females age 18 and over, there were 92.00 males.
The median income for a household in the county was $39,439, and the median income for a family was $46,117. Males had a median income of $33,079 versus $21,774 for females. The per capita income for the county was $18,474. About 4.20% of families and 6.40% of the population were below the poverty line, including 7.90% of those under age 18 and 7.60% of those age 65 or over.