High Quality Content by WIKIPEDIA articles! Jacksonville, North Carolina, is a city in Onslow County, North Carolina, United States. As of the 2010 United States census, the population stood at 70,145, which makes Jacksonville the 14th largest city in North Carolina. Jacksonville is the principal city of and is included in the Jacksonville, North Carolina metropolitan area. Demographically, Jacksonville is the youngest city in the United States with an average age of 22.8 years old, which can be attributed to the large military presence.