Middlesex, NJ has a population of 13,811 and is the 3,029th largest city in the United States. The population density is 3,959 per sq mi which is 395824% higher than the New Jersey average and 4270% higher than the national average. The median age in Middlesex is 39 which is approximately 1% lower than the New Jersey average of 40. In Middlesex, 60% of the population over 15 years of age are married, 72% speak English and 18% speak Spanish. 63% of Middlesex residents were born in New Jersey, 15% were born out of state, 1% were born outside of the United States and 21% were foreign born.