| | Quincy, CA | Twain Harte, CA | United States |
| Population | 1,786 | 2,471 | 329,725,481 |
| Female Population | 58.2% | 41.9% | 50.5% |
| Male Population | 41.8% | 58.1% | 49.5% |
| Median Age | 41.8 | 52.3 | 38.4 |
| space3 | | | |
| Population - 2010 | 1,529 | 2,262 | 309,327,143 |
| Population - 2000 | 1,879 | 2,586 | 285,036,114 |
| Population - 1990 | 1,820 | 2,170 | 251,960,433 |
| Pop. 2000 to Now | 6.0% | 6.0% | 15.6% |
| Pop. 1990 to Now | 9.5% | 26.3% | 30.8% |
| space4 | | | |
| Population Density | 422.8 | 668.0 | 93.4 |
| Land Area | 4.2 | 3.7 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Quincy, CA | Twain Harte, CA | United States |
| Age 0 to 4 | 2.2% | 6.1% | 5.9% |
| Age 5 to 9 | 2.8% | 3.9% | 6.1% |
| Age 10 to 14 | 5.2% | 3.0% | 6.6% |
| Age 15 to 17 | 7.8% | 1.3% | 3.9% |
| Age 18 to 20 | 12.9% | 1.7% | 4.0% |
| Age 21 to 24 | 0.8% | 4.7% | 5.2% |
| Age 25 to 34 | 10.5% | 17.0% | 13.8% |
| Age 35 to 44 | 12.6% | 8.6% | 12.9% |
| Age 45 to 54 | 7.2% | 10.5% | 12.6% |
| Age 55 to 59 | 11.4% | 7.4% | 6.7% |
| Age 60 to 64 | 7.7% | 10.5% | 6.3% |
| Age 65 to 74 | 11.9% | 17.2% | 9.6% |
| Age 75 to 84 | 5.4% | 6.4% | 4.5% |
| Age 85 and over | 1.6% | 1.7% | 1.9% |
| space1 | | | |
| Race | | | |
| | Quincy, CA | Twain Harte, CA | United States |
| White | 78.3% | 83.0% | 59.4% |
| Black | 10.8% | 0.0% | 12.2% |
| Asian | 0.0% | 1.2% | 5.6% |
| Native American | 1.7% | 0.5% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 3.7% | 2.1% | 3.2% |
| Hispanic | 5.6% | 13.2% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Quincy, CA | Twain Harte, CA | United States |
| Married Population | 38.7% | 60.4% | 49.8% |
| Currently Married | 38.7% | 58.3% | 48.0% |
| Married but Separated | 0.0% | 2.1% | 1.8% |
| Single Population | 61.3% | 39.6% | 50.2% |
| Never Married | 46.0% | 21.7% | 33.8% |
| Divorced | 11.5% | 12.5% | 10.8% |
| Widowed | 3.7% | 5.5% | 5.6% |
| space5 | | | |
| Household Size | 2.47 | 2.20 | 2.60 |
| Households | 701 | 1,121 | 124,010,992 |
| Family Households | 386 | 706 | 80,755,759 |
| Married couple, w/children | 29.8% | 29.7% | 30.5% |
| Married couple, no children | 46.4% | 61.0% | 42.9% |
| Non Family Households | 315 | 415 | 43,255,233 |
| Single householder, w/children | 22.0% | 2.1% | 15.8% |
| Single householder, no children | 1.8% | 7.1% | 10.8% |