| | Cos Cob, CT | Summit, NJ | United States |
| Population | 7,089 | 22,598 | 329,725,481 |
| Female Population | 51.1% | 51.9% | 50.5% |
| Male Population | 48.9% | 48.1% | 49.5% |
| Median Age | 41.1 | 39.7 | 38.4 |
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| Population - 2010 | 6,126 | 21,493 | 309,327,143 |
| Population - 2000 | 6,701 | 21,131 | 285,036,114 |
| Population - 1990 | 6,511 | 19,757 | 251,960,433 |
| Pop. 2000 to Now | 1.8% | 3.2% | 15.6% |
| Pop. 1990 to Now | 4.7% | 10.4% | 30.8% |
| space4 | | | |
| Population Density | 3,414.7 | 3,770.1 | 93.4 |
| Land Area | 2.1 | 6.0 | 3,531,905.4 |
| Water Area | 0.1 | 0.1 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Cos Cob, CT | Summit, NJ | United States |
| Age 0 to 4 | 7.0% | 7.4% | 5.9% |
| Age 5 to 9 | 8.2% | 6.2% | 6.1% |
| Age 10 to 14 | 6.5% | 8.6% | 6.6% |
| Age 15 to 17 | 6.8% | 5.5% | 3.9% |
| Age 18 to 20 | 2.5% | 3.3% | 4.0% |
| Age 21 to 24 | 3.0% | 3.6% | 5.2% |
| Age 25 to 34 | 8.8% | 7.7% | 13.8% |
| Age 35 to 44 | 12.8% | 15.5% | 12.9% |
| Age 45 to 54 | 15.3% | 16.4% | 12.6% |
| Age 55 to 59 | 8.5% | 7.5% | 6.7% |
| Age 60 to 64 | 6.0% | 5.8% | 6.3% |
| Age 65 to 74 | 8.6% | 6.2% | 9.6% |
| Age 75 to 84 | 3.2% | 4.1% | 4.5% |
| Age 85 and over | 2.9% | 2.2% | 1.9% |
| space1 | | | |
| Race | | | |
| | Cos Cob, CT | Summit, NJ | United States |
| White | 74.9% | 66.7% | 59.4% |
| Black | 0.3% | 7.0% | 12.2% |
| Asian | 5.4% | 9.1% | 5.6% |
| Native American | 0.2% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.8% | 1.5% | 0.4% |
| Two or More Races | 7.2% | 4.0% | 3.2% |
| Hispanic | 11.2% | 11.7% | 18.4% |
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| Marriage & Family | | | |
| | Cos Cob, CT | Summit, NJ | United States |
| Married Population | 58.5% | 55.8% | 49.8% |
| Currently Married | 57.9% | 55.1% | 48.0% |
| Married but Separated | 0.7% | 0.6% | 1.8% |
| Single Population | 41.5% | 44.2% | 50.2% |
| Never Married | 28.8% | 32.8% | 33.8% |
| Divorced | 9.4% | 6.7% | 10.8% |
| Widowed | 3.4% | 4.7% | 5.6% |
| space5 | | | |
| Household Size | 2.74 | 2.85 | 2.60 |
| Households | 2,572 | 7,853 | 124,010,992 |
| Family Households | 1,902 | 5,687 | 80,755,759 |
| Married couple, w/children | 44.3% | 49.9% | 30.5% |
| Married couple, no children | 38.9% | 31.4% | 42.9% |
| Non Family Households | 670 | 2,166 | 43,255,233 |
| Single householder, w/children | 13.2% | 8.7% | 15.8% |
| Single householder, no children | 3.6% | 10.0% | 10.8% |