Lake Forest, IL has a population of 18,881 and is the 2,314th largest city in the United States. The population density is 1,098 per sq mi which is 394% higher than the Illinois average and 1112% higher than the national average. The median age in Lake Forest is 47 which is approximately 26% higher than the Illinois average of 37. In Lake Forest, 68% of the population over 15 years of age are married, 89% speak English and 2% speak Spanish. 53% of Lake Forest residents were born in Illinois, 35% were born out of state, 2% were born outside of the United States and 11% were foreign born.