| | Darien, CT | San Luis Obispo, CA | United States |
| Population | 21,485 | 47,257 | 329,725,481 |
| Female Population | 50.4% | 49.1% | 50.5% |
| Male Population | 49.6% | 50.9% | 49.5% |
| Median Age | 39.9 | 26 | 38.4 |
| space3 | | | |
| Population - 2010 | 20,752 | 45,193 | 309,327,143 |
| Population - 2000 | 19,607 | 44,174 | 285,036,114 |
| Population - 1990 | 18,196 | 41,958 | 251,960,433 |
| Pop. 2000 to Now | 10.6% | 7.7% | 15.6% |
| Pop. 1990 to Now | 19.2% | 13.4% | 30.8% |
| space4 | | | |
| Population Density | 1,697.9 | 3,597.8 | 93.4 |
| Land Area | 12.7 | 13.1 | 3,531,905.4 |
| Water Area | 10.8 | 0.2 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Darien, CT | San Luis Obispo, CA | United States |
| Age 0 to 4 | 6.4% | 3.1% | 5.9% |
| Age 5 to 9 | 9.3% | 3.1% | 6.1% |
| Age 10 to 14 | 10.4% | 2.9% | 6.6% |
| Age 15 to 17 | 5.9% | 1.9% | 3.9% |
| Age 18 to 20 | 3.4% | 14.7% | 4.0% |
| Age 21 to 24 | 3.3% | 22.3% | 5.2% |
| Age 25 to 34 | 5.3% | 13.4% | 13.8% |
| Age 35 to 44 | 13.8% | 8.4% | 12.9% |
| Age 45 to 54 | 16.4% | 7.5% | 12.6% |
| Age 55 to 59 | 6.2% | 4.1% | 6.7% |
| Age 60 to 64 | 6.4% | 5.5% | 6.3% |
| Age 65 to 74 | 7.7% | 7.8% | 9.6% |
| Age 75 to 84 | 3.1% | 3.7% | 4.5% |
| Age 85 and over | 2.5% | 1.8% | 1.9% |
| space1 | | | |
| Race | | | |
| | Darien, CT | San Luis Obispo, CA | United States |
| White | 84.8% | 70.7% | 59.4% |
| Black | 0.8% | 1.6% | 12.2% |
| Asian | 6.1% | 4.8% | 5.6% |
| Native American | 0.0% | 0.3% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.3% | 0.3% | 0.4% |
| Two or More Races | 3.1% | 3.9% | 3.2% |
| Hispanic | 4.9% | 18.5% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Darien, CT | San Luis Obispo, CA | United States |
| Married Population | 67.1% | 28.8% | 49.8% |
| Currently Married | 65.3% | 27.9% | 48.0% |
| Married but Separated | 1.8% | 0.9% | 1.8% |
| Single Population | 32.9% | 71.2% | 50.2% |
| Never Married | 24.4% | 59.0% | 33.8% |
| Divorced | 4.3% | 8.3% | 10.8% |
| Widowed | 4.2% | 3.9% | 5.6% |
| space5 | | | |
| Household Size | 3.07 | 2.41 | 2.60 |
| Households | 6,978 | 19,035 | 124,010,992 |
| Family Households | 5,743 | 7,693 | 80,755,759 |
| Married couple, w/children | 50.4% | 23.1% | 30.5% |
| Married couple, no children | 38.0% | 49.4% | 42.9% |
| Non Family Households | 1,235 | 11,342 | 43,255,233 |
| Single householder, w/children | 4.7% | 12.1% | 15.8% |
| Single householder, no children | 6.8% | 15.3% | 10.8% |