Mellwood City has a median age of 40.1 years, with a distribution that ranges from under-five to over-sixty. Twenty-nine percent of the population was under the age of eighteen, while 7.3% of residents were between the ages of twenty-four and thirty-four. Forty-two percent of the population was between the ages of 25 and 44, and twenty-one percent of those were 65 years and older. Overall, the gender distribution is about equal, with 49.8% males and 50.2% females.
In 2010, the population of Mellwood City was 15,479, or about a sixth of the city's total population. This makes it one of the more diverse cities in Washington, with a mix of urban and suburban lifestyles. The city is home to West Valley High School, which serves approximately 15,000 residents. The median household income was $58,477. The median household size was $22,690.
Ellwood City is a suburb of Pittsburgh, Pennsylvania. Its population of 7,921 is 9.4% higher than the average for the greater Pittsburgh area. Its population density is 2.07 people per square mile, making it more densely populated than most cities in the country. The average age of the residents of Ellwood City is thirty-one years old, which is significantly higher than the national average.