Jefferson County's population was estimated to be 18,848 in the year 2006, an increase of 381, or +2.1%, over the previous six years.
As of the U.S. Census in 2000, there were 18,426 people, 6,830 households, and 5,190 families residing in the county. The population density was 34 people per square mile (13/km²). There were 7,491 housing units at an average density of 14 per square mile (5/km²). The racial makeup of the county was 96.70% White, 0.92% Native American, 0.37% Black or African American, 0.17% Asian, 0.01% Pacific Islander, 0.42% from other races, and 1.41% from two or more races. Hispanic or Latino of any race were 1.28% of the population.
There were 6,830 households out of which 35.70% had children under the age of 18 living with them, 65.20% were married couples living together, 7.00% had a female householder with no husband present, and 24.00% were non-families. 20.10% of all households were made up of individuals and 9.30% had someone living alone who was 65 years of age or older. The average household size was 2.66 and the average family size was 3.07.
In the county the population was spread out with 27.40% under the age of 18, 7.00% from 18 to 24, 28.00% from 25 to 44, 24.90% from 45 to 64, and 12.80% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 102.60 males. For every 100 females age 18 and over, there were 98.90 males.
The median income for a household in the county was $45,535, and the median income for a family was $50,557. Males had a median income of $36,174 versus $25,468 for females. The per capita income for the county was $19,373. About 5.30% of families and 6.70% of the population were below the poverty line, including 6.90% of those under age 18 and 7.70% of those age 65 or over.