| | Plymouth, NH | Biddeford, ME | United States |
| Population | 6,645 | 22,331 | 329,725,481 |
| Female Population | 47.1% | 50.4% | 50.5% |
| Male Population | 52.9% | 49.6% | 49.5% |
| Median Age | 32 | 36.1 | 38.4 |
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| Population - 2010 | 6,989 | 21,270 | 309,327,143 |
| Population - 2000 | 5,892 | 20,942 | 285,036,114 |
| Population - 1990 | 5,811 | 20,710 | 251,960,433 |
| Pop. 2000 to Now | 17.4% | 2.7% | 15.6% |
| Pop. 1990 to Now | 19.0% | 3.8% | 30.8% |
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| Population Density | 236.3 | 742.3 | 93.4 |
| Land Area | 28.1 | 30.1 | 3,531,905.4 |
| Water Area | 0.6 | 29.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Plymouth, NH | Biddeford, ME | United States |
| Age 0 to 4 | 0.9% | 4.2% | 5.9% |
| Age 5 to 9 | 6.7% | 3.9% | 6.1% |
| Age 10 to 14 | 5.1% | 4.1% | 6.6% |
| Age 15 to 17 | 2.9% | 3.0% | 3.9% |
| Age 18 to 20 | 21.1% | 5.9% | 4.0% |
| Age 21 to 24 | 5.7% | 9.0% | 5.2% |
| Age 25 to 34 | 10.4% | 18.4% | 13.8% |
| Age 35 to 44 | 9.8% | 11.9% | 12.9% |
| Age 45 to 54 | 11.3% | 10.8% | 12.6% |
| Age 55 to 59 | 3.7% | 6.0% | 6.7% |
| Age 60 to 64 | 6.4% | 6.0% | 6.3% |
| Age 65 to 74 | 10.6% | 9.5% | 9.6% |
| Age 75 to 84 | 3.8% | 5.5% | 4.5% |
| Age 85 and over | 1.6% | 1.7% | 1.9% |
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| Race | | | |
| | Plymouth, NH | Biddeford, ME | United States |
| White | 88.3% | 90.7% | 59.4% |
| Black | 2.9% | 1.3% | 12.2% |
| Asian | 2.6% | 2.5% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.1% | 0.4% |
| Two or More Races | 2.1% | 2.6% | 3.2% |
| Hispanic | 4.1% | 2.7% | 18.4% |
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| Marriage & Family | | | |
| | Plymouth, NH | Biddeford, ME | United States |
| Married Population | 34.7% | 40.2% | 49.8% |
| Currently Married | 32.5% | 38.7% | 48.0% |
| Married but Separated | 2.3% | 1.4% | 1.8% |
| Single Population | 65.3% | 59.9% | 50.2% |
| Never Married | 53.0% | 42.9% | 33.8% |
| Divorced | 9.4% | 12.7% | 10.8% |
| Widowed | 2.9% | 4.3% | 5.6% |
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| Household Size | 2.64 | 2.29 | 2.60 |
| Households | 2,061 | 9,198 | 124,010,992 |
| Family Households | 1,271 | 5,094 | 80,755,759 |
| Married couple, w/children | 22.9% | 21.3% | 30.5% |
| Married couple, no children | 38.7% | 48.4% | 42.9% |
| Non Family Households | 790 | 4,104 | 43,255,233 |
| Single householder, w/children | 25.4% | 18.4% | 15.8% |
| Single householder, no children | 13.0% | 11.9% | 10.8% |