Castaic, CA has a population of 19,529 and is the 2,228th largest city in the United States. The population density is 2,689 per sq mi which is 1057% higher than the California average and 2868% higher than the national average. The median age in Castaic is 36 which is approximately equal to the California average of 36. In Castaic, 61% of the population over 15 years of age are married, 76% speak English and 15% speak Spanish. 63% of Castaic residents were born in California, 19% were born out of state, 0% were born outside of the United States and 18% were foreign born.