| | Mill Creek, WA | Naperville, IL | United States |
| Population | 20,665 | 149,013 | 329,725,481 |
| Female Population | 53.1% | 50.5% | 50.5% |
| Male Population | 46.9% | 49.5% | 49.5% |
| Median Age | 39.6 | 39.3 | 38.4 |
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| Population - 2010 | 18,268 | 142,408 | 309,327,143 |
| Population - 2000 | 11,525 | 128,358 | 285,036,114 |
| Population - 1990 | 7,172 | 85,351 | 251,960,433 |
| Pop. 2000 to Now | 80.5% | 15.3% | 15.6% |
| Pop. 1990 to Now | 190.0% | 73.4% | 30.8% |
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| Population Density | 4,488.5 | 3,839.6 | 93.4 |
| Land Area | 4.6 | 38.8 | 3,531,905.4 |
| Water Area | 0.0 | 0.6 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Mill Creek, WA | Naperville, IL | United States |
| Age 0 to 4 | 5.5% | 6.0% | 5.9% |
| Age 5 to 9 | 5.9% | 6.5% | 6.1% |
| Age 10 to 14 | 6.8% | 7.8% | 6.6% |
| Age 15 to 17 | 3.3% | 4.9% | 3.9% |
| Age 18 to 20 | 2.4% | 3.2% | 4.0% |
| Age 21 to 24 | 4.8% | 4.2% | 5.2% |
| Age 25 to 34 | 14.7% | 11.3% | 13.8% |
| Age 35 to 44 | 13.8% | 14.2% | 12.9% |
| Age 45 to 54 | 13.9% | 15.0% | 12.6% |
| Age 55 to 59 | 7.3% | 7.6% | 6.7% |
| Age 60 to 64 | 5.9% | 5.9% | 6.3% |
| Age 65 to 74 | 8.7% | 8.3% | 9.6% |
| Age 75 to 84 | 4.9% | 3.5% | 4.5% |
| Age 85 and over | 2.0% | 1.5% | 1.9% |
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| Race | | | |
| | Mill Creek, WA | Naperville, IL | United States |
| White | 60.2% | 65.1% | 59.4% |
| Black | 4.3% | 4.3% | 12.2% |
| Asian | 20.0% | 20.6% | 5.6% |
| Native American | 0.4% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.8% | 0.0% | 0.2% |
| Other | 0.5% | 0.3% | 0.4% |
| Two or More Races | 6.3% | 3.2% | 3.2% |
| Hispanic | 7.6% | 6.5% | 18.4% |
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| Marriage & Family | | | |
| | Mill Creek, WA | Naperville, IL | United States |
| Married Population | 58.6% | 61.8% | 49.8% |
| Currently Married | 57.5% | 60.9% | 48.0% |
| Married but Separated | 1.2% | 0.9% | 1.8% |
| Single Population | 41.4% | 38.2% | 50.2% |
| Never Married | 26.9% | 27.2% | 33.8% |
| Divorced | 10.2% | 7.0% | 10.8% |
| Widowed | 4.2% | 4.0% | 5.6% |
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| Household Size | 2.42 | 2.75 | 2.60 |
| Households | 8,492 | 53,234 | 124,010,992 |
| Family Households | 5,410 | 39,815 | 80,755,759 |
| Married couple, w/children | 40.8% | 42.3% | 30.5% |
| Married couple, no children | 44.8% | 43.9% | 42.9% |
| Non Family Households | 3,082 | 13,419 | 43,255,233 |
| Single householder, w/children | 7.4% | 8.4% | 15.8% |
| Single householder, no children | 7.0% | 5.3% | 10.8% |