Located along the Front Range of the Rocky Mountains, Jefferson County is part of the Denver metropolitan area. The major expressway Interstate 70 traverses Jefferson County from east-to-west, passing just south of the county seat of Golden. Jefferson County is a quite long county from north to south, following the Front Range. The county is known as the Gateway to the Rocky Mountains.
A major employer in Jefferson County is the large Coors Brewing Company in Golden. Also, the state-supported Colorado School of Mines is located in Jefferson County, offering programs in mining and engineering.
Jefferson County is commonly nicknamed Jeffco. The name Jeffco is incorporated in the name of the Jeffco School District, the Jeffco Business Center Metropolitan District No. 1, and several businesses located in Jefferson County. Jeffco is also incorporated in the unofficial monikers of many Jefferson County agencies. The Rocky Mountain Metropolitan Airport operated by Jefferson County was previously known as the Jeffco Airport.
As of the census of 2000, there were 527,056 people, 206,067 households, and 140,537 families residing in the county. The population density was 683 people per square mile (264/km²). There were 212,488 housing units at an average density of 275 per square mile (106/km²). The racial makeup of the county was 90.59% White, 0.89% Black or African American, 0.75% Native American, 2.28% Asian, 0.08% Pacific Islander, 3.23% from other races, and 2.18% from two or more races. 9.95% of the population were Hispanic or Latino of any race.
There were 206,067 households out of which 33.40% had children under the age of 18 living with them, 55.10% were married couples living together, 9.10% had a female householder with no husband present, and 31.80% were non-families. 24.50% of all households were made up of individuals and 6.30% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 3.03.
In the county the population was spread out with 25.30% under the age of 18, 8.10% from 18 to 24, 32.10% from 25 to 44, 24.90% from 45 to 64, and 9.60% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 99.00 males. For every 100 females age 18 and over, there were 96.80 males.
The median income for a household in the county was $57,339, and the median income for a family was $67,310. Males had a median income of $45,306 versus $32,372 for females. The per capita income for the county was $28,066. About 3.40% of families and 5.20% of the population were below the poverty line, including 5.80% of those under age 18 and 5.10% of those age 65 or over.