Jacksonville is a city in Onslow County, North Carolina, United States. As of the 2020 United States census, the population was 72,723, which makes Jacksonville the 14th-largest city in North Carolina. Jacksonville is the county seat and most populous community of Onslow County, which is coterminous with 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. The low age may also be in part due to the population drastically going up over the past 80 years, from 783 in the 1930 census to 72,876 in the 2021 Census estimate.