| | St. Joseph, IL | St. Charles, IL | United States |
| Population | 3,888 | 32,908 | 329,725,481 |
| Female Population | 54.3% | 50.5% | 50.5% |
| Male Population | 45.7% | 49.5% | 49.5% |
| Median Age | 39.4 | 42.2 | 38.4 |
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| Population - 2010 | 3,994 | 32,323 | 309,327,143 |
| Population - 2000 | 2,912 | 27,896 | 285,036,114 |
| Population - 1990 | 2,052 | 22,501 | 251,960,433 |
| Pop. 2000 to Now | 30.5% | 17.3% | 15.6% |
| Pop. 1990 to Now | 85.2% | 45.4% | 30.8% |
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| Population Density | 1,866.5 | 2,285.1 | 93.4 |
| Land Area | 2.1 | 14.4 | 3,531,905.4 |
| Water Area | 0.0 | 0.6 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | St. Joseph, IL | St. Charles, IL | United States |
| Age 0 to 4 | 5.4% | 4.6% | 5.9% |
| Age 5 to 9 | 3.8% | 4.7% | 6.1% |
| Age 10 to 14 | 11.5% | 5.8% | 6.6% |
| Age 15 to 17 | 6.9% | 5.2% | 3.9% |
| Age 18 to 20 | 1.2% | 2.9% | 4.0% |
| Age 21 to 24 | 5.7% | 7.2% | 5.2% |
| Age 25 to 34 | 8.3% | 10.8% | 13.8% |
| Age 35 to 44 | 15.9% | 11.9% | 12.9% |
| Age 45 to 54 | 6.5% | 15.9% | 12.6% |
| Age 55 to 59 | 5.0% | 7.9% | 6.7% |
| Age 60 to 64 | 13.9% | 6.2% | 6.3% |
| Age 65 to 74 | 11.3% | 9.8% | 9.6% |
| Age 75 to 84 | 3.4% | 5.3% | 4.5% |
| Age 85 and over | 1.2% | 1.8% | 1.9% |
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| Race | | | |
| | St. Joseph, IL | St. Charles, IL | United States |
| White | 95.0% | 78.9% | 59.4% |
| Black | 0.1% | 3.5% | 12.2% |
| Asian | 2.5% | 5.6% | 5.6% |
| Native American | 0.2% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 1.0% | 2.7% | 3.2% |
| Hispanic | 1.3% | 9.1% | 18.4% |
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| Marriage & Family | | | |
| | St. Joseph, IL | St. Charles, IL | United States |
| Married Population | 54.7% | 57.0% | 49.8% |
| Currently Married | 53.9% | 56.4% | 48.0% |
| Married but Separated | 0.8% | 0.5% | 1.8% |
| Single Population | 45.3% | 43.0% | 50.2% |
| Never Married | 28.0% | 29.4% | 33.8% |
| Divorced | 8.8% | 8.3% | 10.8% |
| Widowed | 8.5% | 5.3% | 5.6% |
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| Household Size | 2.71 | 2.49 | 2.60 |
| Households | 1,431 | 13,031 | 124,010,992 |
| Family Households | 1,021 | 9,100 | 80,755,759 |
| Married couple, w/children | 37.8% | 34.1% | 30.5% |
| Married couple, no children | 35.5% | 49.1% | 42.9% |
| Non Family Households | 410 | 3,931 | 43,255,233 |
| Single householder, w/children | 23.4% | 9.8% | 15.8% |
| Single householder, no children | 3.3% | 7.0% | 10.8% |