| | Brooklyn, IL | Alton, IL | United States |
| Population | 627 | 25,902 | 329,725,481 |
| Female Population | 61.7% | 52.2% | 50.5% |
| Male Population | 38.3% | 47.8% | 49.5% |
| Median Age | 29.7 | 38.1 | 38.4 |
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| Population - 2010 | 743 | 27,901 | 309,327,143 |
| Population - 2000 | 676 | 30,496 | 285,036,114 |
| Population - 1990 | 1,144 | 32,905 | 251,960,433 |
| Pop. 2000 to Now | 1.6% | -14.6% | 15.6% |
| Pop. 1990 to Now | -40.0% | -20.8% | 30.8% |
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| Population Density | 754.5 | 1,669.5 | 93.4 |
| Land Area | 0.8 | 15.5 | 3,531,905.4 |
| Water Area | 0.0 | 1.3 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Brooklyn, IL | Alton, IL | United States |
| Age 0 to 4 | 9.4% | 5.5% | 5.9% |
| Age 5 to 9 | 14.4% | 7.9% | 6.1% |
| Age 10 to 14 | 6.9% | 7.2% | 6.6% |
| Age 15 to 17 | 0.0% | 3.6% | 3.9% |
| Age 18 to 20 | 0.5% | 3.5% | 4.0% |
| Age 21 to 24 | 3.2% | 3.0% | 5.2% |
| Age 25 to 34 | 20.4% | 14.5% | 13.8% |
| Age 35 to 44 | 9.1% | 13.5% | 12.9% |
| Age 45 to 54 | 10.4% | 11.7% | 12.6% |
| Age 55 to 59 | 7.2% | 6.0% | 6.7% |
| Age 60 to 64 | 2.2% | 7.3% | 6.3% |
| Age 65 to 74 | 13.4% | 10.4% | 9.6% |
| Age 75 to 84 | 1.4% | 4.4% | 4.5% |
| Age 85 and over | 1.6% | 1.6% | 1.9% |
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| Race | | | |
| | Brooklyn, IL | Alton, IL | United States |
| White | 1.3% | 67.1% | 59.4% |
| Black | 91.5% | 25.1% | 12.2% |
| Asian | 0.0% | 0.6% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.2% | 0.4% |
| Two or More Races | 2.4% | 5.0% | 3.2% |
| Hispanic | 4.8% | 1.9% | 18.4% |
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| Marriage & Family | | | |
| | Brooklyn, IL | Alton, IL | United States |
| Married Population | 12.6% | 36.6% | 49.8% |
| Currently Married | 11.3% | 33.8% | 48.0% |
| Married but Separated | 1.4% | 2.8% | 1.8% |
| Single Population | 87.4% | 63.4% | 50.2% |
| Never Married | 58.9% | 37.6% | 33.8% |
| Divorced | 20.7% | 17.2% | 10.8% |
| Widowed | 7.8% | 8.7% | 5.6% |
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| Household Size | 3.46 | 2.29 | 2.60 |
| Households | 181 | 11,097 | 124,010,992 |
| Family Households | 108 | 5,853 | 80,755,759 |
| Married couple, w/children | 0.0% | 23.2% | 30.5% |
| Married couple, no children | 13.0% | 32.2% | 42.9% |
| Non Family Households | 73 | 5,244 | 43,255,233 |
| Single householder, w/children | 68.5% | 31.4% | 15.8% |
| Single householder, no children | 18.5% | 13.3% | 10.8% |