Allendale, IL has a population of 666 and is the 17,767th largest city in the United States. The population density is 2,159 per sq mi which is 847% higher than the Illinois average and 2283% higher than the national average. The median age in Allendale is 35 which is approximately 6% lower than the Illinois average of 37. In Allendale, 61% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 55% of Allendale residents were born in Illinois, 44% were born out of state, 0% were born outside of the United States and 1% were foreign born.