Thornton, CO has a population of 130,511 and is the 203rd largest city in the United States. The population density is 3,617 per sq mi which is 7165% higher than the Colorado average and 3892% higher than the national average. The median age in Thornton is 33 which is approximately 9% lower than the Colorado average of 36. In Thornton, 57% of the population over 15 years of age are married, 77% speak English and 17% speak Spanish. 53% of Thornton residents were born in Colorado, 34% were born out of state, 1% were born outside of the United States and 13% were foreign born.