The median age in the US is 37.3 years old according to the 2013 American Communities Survey published by the US Census Bureau. The state with the youngest median age is Utah, were the average resident is 29.1 years old. Maine has the oldest residents with a median age of 43.2 years old.
The information displayed above comes from the American Communities Survey with is published by the US Census Bureau. These figures can be found on Table S0101, 2009-2013 Five Year Estimates at http://factfinder.census.gov/. The variable used is Total, Median Age.