New Castle, CO has a population of 4,610 and is the 6,822nd largest city in the United States. The population density is 1,735 per sq mi which is 2998% higher than the Colorado average and 1815% higher than the national average. The median age in New Castle is 33 which is approximately 11% lower than the Colorado average of 36. In New Castle, 63% of the population over 15 years of age are married, 67% speak English and 29% speak Spanish. 44% of New Castle residents were born in Colorado, 41% were born out of state, 1% were born outside of the United States and 14% were foreign born.