Jefferson, IA has a population of 4,207 and is the 7,214th largest city in the United States. The population density is 705 per sq mi which is 1190% higher than the Iowa average and 678% higher than the national average. The median age in Jefferson is 48 which is approximately 27% higher than the Iowa average of 38. In Jefferson, 68% of the population over 15 years of age are married, 97% speak English and 2% speak Spanish. 77% of Jefferson residents were born in Iowa, 21% were born out of state, 1% were born outside of the United States and 2% were foreign born.