| | Jacksonville, IL | Hartford, CT | United States |
| Population | 18,114 | 121,562 | 329,725,481 |
| Female Population | 49.7% | 52.1% | 50.5% |
| Male Population | 50.3% | 47.9% | 49.5% |
| Median Age | 37.4 | 33 | 38.4 |
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| Population - 2010 | 19,418 | 125,349 | 309,327,143 |
| Population - 2000 | 18,940 | 121,578 | 285,036,114 |
| Population - 1990 | 19,324 | 139,739 | 251,960,433 |
| Pop. 2000 to Now | -2.4% | 0.0% | 15.6% |
| Pop. 1990 to Now | -4.3% | -13.0% | 30.8% |
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| Population Density | 1,696.4 | 6,994.8 | 93.4 |
| Land Area | 10.7 | 17.4 | 3,531,905.4 |
| Water Area | 0.2 | 0.7 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Jacksonville, IL | Hartford, CT | United States |
| Age 0 to 4 | 5.4% | 5.7% | 5.9% |
| Age 5 to 9 | 5.1% | 6.7% | 6.1% |
| Age 10 to 14 | 4.3% | 7.0% | 6.6% |
| Age 15 to 17 | 3.0% | 4.0% | 3.9% |
| Age 18 to 20 | 8.0% | 7.4% | 4.0% |
| Age 21 to 24 | 6.1% | 6.6% | 5.2% |
| Age 25 to 34 | 15.1% | 15.7% | 13.8% |
| Age 35 to 44 | 11.8% | 13.2% | 12.9% |
| Age 45 to 54 | 9.5% | 11.9% | 12.6% |
| Age 55 to 59 | 6.9% | 5.3% | 6.7% |
| Age 60 to 64 | 6.0% | 5.1% | 6.3% |
| Age 65 to 74 | 9.3% | 7.2% | 9.6% |
| Age 75 to 84 | 5.3% | 2.9% | 4.5% |
| Age 85 and over | 4.1% | 1.4% | 1.9% |
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| Race | | | |
| | Jacksonville, IL | Hartford, CT | United States |
| White | 81.6% | 14.7% | 59.4% |
| Black | 9.8% | 34.1% | 12.2% |
| Asian | 0.9% | 2.6% | 5.6% |
| Native American | 0.3% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.1% | 0.6% | 0.4% |
| Two or More Races | 3.3% | 2.5% | 3.2% |
| Hispanic | 4.1% | 45.5% | 18.4% |
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| Marriage & Family | | | |
| | Jacksonville, IL | Hartford, CT | United States |
| Married Population | 37.0% | 27.4% | 49.8% |
| Currently Married | 35.4% | 24.9% | 48.0% |
| Married but Separated | 1.7% | 2.5% | 1.8% |
| Single Population | 63.0% | 72.6% | 50.2% |
| Never Married | 39.6% | 56.8% | 33.8% |
| Divorced | 16.2% | 11.4% | 10.8% |
| Widowed | 7.1% | 4.4% | 5.6% |
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| Household Size | 2.20 | 2.43 | 2.60 |
| Households | 6,935 | 46,879 | 124,010,992 |
| Family Households | 3,770 | 26,099 | 80,755,759 |
| Married couple, w/children | 20.4% | 20.1% | 30.5% |
| Married couple, no children | 46.7% | 19.3% | 42.9% |
| Non Family Households | 3,165 | 20,780 | 43,255,233 |
| Single householder, w/children | 20.1% | 37.6% | 15.8% |
| Single householder, no children | 12.9% | 23.1% | 10.8% |