Castle Pines, CO has a population of 10,389 and is the 3,839th largest city in the United States. The population density is 1,083 per sq mi which is 2075% higher than the Colorado average and 1095% higher than the national average. The median age in Castle Pines is 42 which is approximately 16% higher than the Colorado average of 36. In Castle Pines, 74% of the population over 15 years of age are married, 92% speak English and 4% speak Spanish. 31% of Castle Pines residents were born in Colorado, 62% were born out of state, 2% were born outside of the United States and 5% were foreign born.