| | St. Charles, ID | Merrimack, NH | United States |
| Population | 117 | 26,762 | 329,725,481 |
| Female Population | 31.6% | 50.0% | 50.5% |
| Male Population | 68.4% | 50.0% | 49.5% |
| Median Age | 58.1 | 42.1 | 38.4 |
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| Population - 2010 | 143 | 25,507 | 309,327,143 |
| Population - 2000 | 156 | 25,119 | 285,036,114 |
| Population - 1990 | 189 | 22,156 | 251,960,433 |
| Pop. 2000 to Now | -1.9% | 9.3% | 15.6% |
| Pop. 1990 to Now | -19.1% | 23.9% | 30.8% |
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| Population Density | 167.4 | 822.7 | 93.4 |
| Land Area | 0.7 | 32.5 | 3,531,905.4 |
| Water Area | 0.0 | 0.9 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | St. Charles, ID | Merrimack, NH | United States |
| Age 0 to 4 | 6.0% | 5.2% | 5.9% |
| Age 5 to 9 | 0.0% | 4.9% | 6.1% |
| Age 10 to 14 | 0.0% | 8.3% | 6.6% |
| Age 15 to 17 | 7.7% | 4.4% | 3.9% |
| Age 18 to 20 | 14.5% | 3.5% | 4.0% |
| Age 21 to 24 | 5.1% | 3.2% | 5.2% |
| Age 25 to 34 | 0.0% | 12.7% | 13.8% |
| Age 35 to 44 | 0.0% | 12.0% | 12.9% |
| Age 45 to 54 | 6.8% | 13.8% | 12.6% |
| Age 55 to 59 | 19.7% | 8.3% | 6.7% |
| Age 60 to 64 | 0.9% | 6.8% | 6.3% |
| Age 65 to 74 | 35.0% | 11.2% | 9.6% |
| Age 75 to 84 | 4.3% | 4.7% | 4.5% |
| Age 85 and over | 0.0% | 1.0% | 1.9% |
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| Race | | | |
| | St. Charles, ID | Merrimack, NH | United States |
| White | 88.0% | 86.4% | 59.4% |
| Black | 0.0% | 1.1% | 12.2% |
| Asian | 5.1% | 4.4% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.4% | 0.4% |
| Two or More Races | 0.0% | 2.7% | 3.2% |
| Hispanic | 6.8% | 5.0% | 18.4% |
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| Marriage & Family | | | |
| | St. Charles, ID | Merrimack, NH | United States |
| Married Population | 59.1% | 58.0% | 49.8% |
| Currently Married | 59.1% | 57.4% | 48.0% |
| Married but Separated | 0.0% | 0.6% | 1.8% |
| Single Population | 40.9% | 42.0% | 50.2% |
| Never Married | 18.2% | 27.5% | 33.8% |
| Divorced | 19.1% | 10.7% | 10.8% |
| Widowed | 3.6% | 3.8% | 5.6% |
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| Household Size | 2.13 | 2.66 | 2.60 |
| Households | 55 | 10,009 | 124,010,992 |
| Family Households | 30 | 7,271 | 80,755,759 |
| Married couple, w/children | 36.7% | 31.9% | 30.5% |
| Married couple, no children | 63.3% | 52.7% | 42.9% |
| Non Family Households | 25 | 2,738 | 43,255,233 |
| Single householder, w/children | 0.0% | 9.0% | 15.8% |
| Single householder, no children | 0.0% | 6.4% | 10.8% |