Declo, ID has a population of 510 and is the 19,413th largest city in the United States. The population density is 1,843 per sq mi which is 9558% higher than the Idaho average and 1934% higher than the national average. The median age in Declo is 28 which is approximately 21% lower than the Idaho average of 36. In Declo, 63% of the population over 15 years of age are married, 78% speak English and 19% speak Spanish. 58% of Declo residents were born in Idaho, 28% were born out of state, 0% were born outside of the United States and 14% were foreign born.