Clayton, IL has a population of 607 and is the 18,376th largest city in the United States. The population density is 699 per sq mi which is 214% higher than the Illinois average and 671% higher than the national average. The median age in Clayton is 40 which is approximately 7% higher than the Illinois average of 37. In Clayton, 51% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 88% of Clayton residents were born in Illinois, 12% were born out of state, 0% were born outside of the United States and 0% were foreign born.