St. Clair County is a county located in the U.S. state of Illinois, and determined by the U.S. Census Bureau to include the mean center of U.S. population in 1970. As of 2006, the population was 260,919. Its county seat is Belleville, Illinois.
It is a part of the St. Louis Metropolitan Area. St. Clair County is also the first county established in the state of Illinois. Its existence actually precedes the state of Illinois as it was established in the Northwest Territory in 1790. The county was named in honor of Arthur St. Clair, Governor of the Northwest Territory, who organized the county in 1790. In its original boundaries, the county covered two-thirds of present-day Illinois.
1This is not a county, but an independent city which is not part of a county.
There were 96,810 households out of which 34.50% had children under the age of 18 living with them, 48.10% were married couples living together, 17.10% had a female householder with no husband present, and 30.50% were non-families. 25.90% of all households were made up of individuals and 10.30% 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 3.13.
In the county the population was spread out with 27.70% under the age of 18, 8.90% from 18 to 24, 29.20% from 25 to 44, 21.10% from 45 to 64, and 13.20% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 91.40 males. For every 100 females age 18 and over, there were 87.20 males.
The median income for a household in the county was $39,148, and the median income for a family was $47,409. Males had a median income of $36,569 versus $25,773 for females. The per capita income for the county was $18,932. About 11.80% of families and 14.50% of the population were below the poverty line, including 21.60% of those under age 18 and 9.80% of those age 65 or over.