Plains, PA has a population of 3,615 and is the 7,898th largest city in the United States. The population density is 2,827 per sq mi which is 920% higher than the Pennsylvania average and 3020% higher than the national average. The median age in Plains is 52 which is approximately 27% higher than the Pennsylvania average of 41. In Plains, 57% of the population over 15 years of age are married, 96% speak English and 1% speak Spanish. 86% of Plains residents were born in Pennsylvania, 11% were born out of state, 1% were born outside of the United States and 2% were foreign born.