| | O'Fallon, MO | Lemont, IL | United States |
| Population | 90,802 | 17,535 | 329,725,481 |
| Female Population | 50.7% | 49.3% | 50.5% |
| Male Population | 49.3% | 50.7% | 49.5% |
| Median Age | 37.2 | 44 | 38.4 |
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| Population - 2010 | 79,969 | 16,075 | 309,327,143 |
| Population - 2000 | 46,169 | 13,098 | 285,036,114 |
| Population - 1990 | 18,698 | 7,348 | 251,960,433 |
| Pop. 2000 to Now | 94.4% | 32.4% | 15.6% |
| Pop. 1990 to Now | 380.1% | 136.0% | 30.8% |
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| Population Density | 3,057.0 | 2,167.5 | 93.4 |
| Land Area | 29.7 | 8.1 | 3,531,905.4 |
| Water Area | 0.0 | 0.4 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | O'Fallon, MO | Lemont, IL | United States |
| Age 0 to 4 | 6.8% | 4.0% | 5.9% |
| Age 5 to 9 | 7.0% | 8.6% | 6.1% |
| Age 10 to 14 | 7.5% | 7.5% | 6.6% |
| Age 15 to 17 | 4.9% | 4.7% | 3.9% |
| Age 18 to 20 | 3.1% | 3.2% | 4.0% |
| Age 21 to 24 | 3.9% | 3.6% | 5.2% |
| Age 25 to 34 | 13.5% | 7.4% | 13.8% |
| Age 35 to 44 | 13.9% | 12.7% | 12.9% |
| Age 45 to 54 | 14.5% | 13.9% | 12.6% |
| Age 55 to 59 | 6.8% | 8.9% | 6.7% |
| Age 60 to 64 | 6.1% | 6.4% | 6.3% |
| Age 65 to 74 | 7.1% | 9.1% | 9.6% |
| Age 75 to 84 | 3.7% | 6.2% | 4.5% |
| Age 85 and over | 1.2% | 3.7% | 1.9% |
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| Race | | | |
| | O'Fallon, MO | Lemont, IL | United States |
| White | 84.5% | 89.1% | 59.4% |
| Black | 3.8% | 0.5% | 12.2% |
| Asian | 4.3% | 2.0% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.4% | 0.0% | 0.4% |
| Two or More Races | 2.5% | 1.4% | 3.2% |
| Hispanic | 4.4% | 7.0% | 18.4% |
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| Marriage & Family | | | |
| | O'Fallon, MO | Lemont, IL | United States |
| Married Population | 59.0% | 61.6% | 49.8% |
| Currently Married | 58.1% | 60.7% | 48.0% |
| Married but Separated | 0.8% | 0.9% | 1.8% |
| Single Population | 41.0% | 38.4% | 50.2% |
| Never Married | 25.8% | 24.8% | 33.8% |
| Divorced | 10.4% | 7.5% | 10.8% |
| Widowed | 4.9% | 6.2% | 5.6% |
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| Household Size | 2.73 | 2.83 | 2.60 |
| Households | 33,163 | 6,119 | 124,010,992 |
| Family Households | 24,661 | 4,736 | 80,755,759 |
| Married couple, w/children | 39.1% | 40.0% | 30.5% |
| Married couple, no children | 41.3% | 46.0% | 42.9% |
| Non Family Households | 8,502 | 1,383 | 43,255,233 |
| Single householder, w/children | 14.0% | 8.7% | 15.8% |
| Single householder, no children | 5.5% | 5.3% | 10.8% |