| | Mountainside, NJ | Roselle Park, NJ | United States |
| Population | 7,003 | 13,973 | 329,725,481 |
| Female Population | 51.1% | 50.8% | 50.5% |
| Male Population | 48.9% | 49.2% | 49.5% |
| Median Age | 51 | 37.8 | 38.4 |
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| Population - 2010 | 6,668 | 13,311 | 309,327,143 |
| Population - 2000 | 6,602 | 13,281 | 285,036,114 |
| Population - 1990 | 6,657 | 12,805 | 251,960,433 |
| Pop. 2000 to Now | 4.2% | 1.7% | 15.6% |
| Pop. 1990 to Now | 3.3% | 5.5% | 30.8% |
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| Population Density | 1,752.5 | 11,406.5 | 93.4 |
| Land Area | 4.0 | 1.2 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Mountainside, NJ | Roselle Park, NJ | United States |
| Age 0 to 4 | 9.8% | 5.9% | 5.9% |
| Age 5 to 9 | 4.0% | 5.1% | 6.1% |
| Age 10 to 14 | 4.7% | 7.3% | 6.6% |
| Age 15 to 17 | 4.8% | 3.1% | 3.9% |
| Age 18 to 20 | 1.2% | 3.3% | 4.0% |
| Age 21 to 24 | 2.9% | 3.7% | 5.2% |
| Age 25 to 34 | 3.6% | 16.8% | 13.8% |
| Age 35 to 44 | 11.2% | 14.6% | 12.9% |
| Age 45 to 54 | 12.9% | 13.2% | 12.6% |
| Age 55 to 59 | 9.1% | 8.2% | 6.7% |
| Age 60 to 64 | 8.2% | 6.1% | 6.3% |
| Age 65 to 74 | 13.6% | 7.9% | 9.6% |
| Age 75 to 84 | 8.7% | 3.5% | 4.5% |
| Age 85 and over | 5.2% | 1.6% | 1.9% |
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| Race | | | |
| | Mountainside, NJ | Roselle Park, NJ | United States |
| White | 74.2% | 39.8% | 59.4% |
| Black | 2.3% | 8.8% | 12.2% |
| Asian | 9.9% | 12.3% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 4.3% | 0.4% |
| Two or More Races | 1.0% | 1.3% | 3.2% |
| Hispanic | 12.5% | 33.6% | 18.4% |
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| Marriage & Family | | | |
| | Mountainside, NJ | Roselle Park, NJ | United States |
| Married Population | 70.7% | 51.1% | 49.8% |
| Currently Married | 70.7% | 49.1% | 48.0% |
| Married but Separated | 0.0% | 2.0% | 1.8% |
| Single Population | 29.3% | 48.9% | 50.2% |
| Never Married | 18.8% | 36.1% | 33.8% |
| Divorced | 2.2% | 8.3% | 10.8% |
| Widowed | 8.3% | 4.4% | 5.6% |
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| Household Size | 2.68 | 2.68 | 2.60 |
| Households | 2,531 | 5,204 | 124,010,992 |
| Family Households | 1,983 | 3,585 | 80,755,759 |
| Married couple, w/children | 40.1% | 31.4% | 30.5% |
| Married couple, no children | 55.2% | 38.0% | 42.9% |
| Non Family Households | 548 | 1,619 | 43,255,233 |
| Single householder, w/children | 3.2% | 13.8% | 15.8% |
| Single householder, no children | 1.6% | 16.8% | 10.8% |