| | Middlesex, NJ | South Amboy, NJ | United States |
| Population | 14,513 | 9,320 | 329,725,481 |
| Female Population | 51.5% | 52.5% | 50.5% |
| Male Population | 48.5% | 47.5% | 49.5% |
| Median Age | 42.3 | 41.4 | 38.4 |
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| Population - 2010 | 13,628 | 8,570 | 309,327,143 |
| Population - 2000 | 13,717 | 7,913 | 285,036,114 |
| Population - 1990 | 13,055 | 7,863 | 251,960,433 |
| Pop. 2000 to Now | -1.3% | 14.9% | 15.6% |
| Pop. 1990 to Now | 3.7% | 15.7% | 30.8% |
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| Population Density | 4,160.8 | 6,059.8 | 93.4 |
| Land Area | 3.5 | 1.5 | 3,531,905.4 |
| Water Area | 0.0 | 1.1 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Middlesex, NJ | South Amboy, NJ | United States |
| Age 0 to 4 | 5.4% | 4.2% | 5.9% |
| Age 5 to 9 | 6.1% | 4.2% | 6.1% |
| Age 10 to 14 | 7.0% | 5.2% | 6.6% |
| Age 15 to 17 | 3.8% | 1.2% | 3.9% |
| Age 18 to 20 | 2.5% | 4.1% | 4.0% |
| Age 21 to 24 | 3.3% | 5.2% | 5.2% |
| Age 25 to 34 | 13.2% | 14.8% | 13.8% |
| Age 35 to 44 | 11.6% | 15.7% | 12.9% |
| Age 45 to 54 | 17.0% | 12.0% | 12.6% |
| Age 55 to 59 | 7.8% | 11.0% | 6.7% |
| Age 60 to 64 | 6.6% | 8.4% | 6.3% |
| Age 65 to 74 | 9.1% | 11.0% | 9.6% |
| Age 75 to 84 | 5.3% | 2.3% | 4.5% |
| Age 85 and over | 1.3% | 0.8% | 1.9% |
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| Race | | | |
| | Middlesex, NJ | South Amboy, NJ | United States |
| White | 57.1% | 63.0% | 59.4% |
| Black | 4.7% | 10.3% | 12.2% |
| Asian | 7.3% | 2.6% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.2% | 0.0% | 0.4% |
| Two or More Races | 1.2% | 1.8% | 3.2% |
| Hispanic | 29.5% | 22.2% | 18.4% |
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| Marriage & Family | | | |
| | Middlesex, NJ | South Amboy, NJ | United States |
| Married Population | 58.7% | 53.2% | 49.8% |
| Currently Married | 56.8% | 51.9% | 48.0% |
| Married but Separated | 1.9% | 1.3% | 1.8% |
| Single Population | 41.3% | 46.8% | 50.2% |
| Never Married | 28.2% | 32.6% | 33.8% |
| Divorced | 9.0% | 9.5% | 10.8% |
| Widowed | 4.1% | 4.7% | 5.6% |
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| Household Size | 2.65 | 2.41 | 2.60 |
| Households | 5,459 | 3,857 | 124,010,992 |
| Family Households | 4,147 | 2,750 | 80,755,759 |
| Married couple, w/children | 33.9% | 21.9% | 30.5% |
| Married couple, no children | 43.0% | 47.9% | 42.9% |
| Non Family Households | 1,312 | 1,107 | 43,255,233 |
| Single householder, w/children | 12.2% | 14.4% | 15.8% |
| Single householder, no children | 10.9% | 15.7% | 10.8% |