Keystone, IA has a population of 631 and is the 18,120th largest city in the United States. The population density is 1,398 per sq mi which is 2459% higher than the Iowa average and 1443% higher than the national average. The median age in Keystone is 47 which is approximately 23% higher than the Iowa average of 38. In Keystone, 77% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 89% of Keystone residents were born in Iowa, 10% were born out of state, 0% were born outside of the United States and 1% were foreign born.