The Celina Micropolitan Statistical Area includes all of Mercer County.
2000 40,924 1990 39,443 1980 38,334 1970 35,265 1960 32,559 1950 28,311 1940 26,256 1930 25,096 1920 26,872 1910 27,536 1900 28,021 1890 27,220 1880 21,808 1870 17,254 1860 14,104 1850 7,712 1840 8,277 1830 1,110 1820 95
As of the census of 2000, there were 40,924 people, 14,756 households, and 11,022 families residing in the county. The population density was 88 people per square mile (34/km²). There were 15,875 housing units at an average density of 34 per square mile (13/km²). The racial makeup of the county was 98.44% White, 0.10% Black or African American, 0.26% Native American, 0.29% Asian, 0.02% Pacific Islander, 0.34% from other races, and 0.56% from two or more races. 1.15% of the population were Hispanic or Latino of any race.
There were 14,756 households out of which 37.10% had children under the age of 18 living with them, 64.10% were married couples living together, 7.40% had a female householder with no husband present, and 25.30% were non-families. 22.70% of all households were made up of individuals and 10.80% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.24.
In the county, the population was spread out with 29.60% under the age of 18, 7.90% from 18 to 24, 26.70% from 25 to 44, 21.20% from 45 to 64, and 14.50% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 99.80 males. For every 100 females age 18 and over, there were 96.20 males.
The median income for a household in the county was $42,742, and the median income for a family was $50,157. Males had a median income of $35,508 versus $22,857 for females. The per capita income for the county was $18,531. About 4.60% of families and 6.40% of the population were below the poverty line, including 6.90% of those under age 18 and 7.80% of those age 65 or over.