We lack really solid data to give an exact answer for the period before 1800. Here is the median age for white people in the United States between 1800 and 1830 (Michael R. Haines, Richard H. Steckel, eds. A Population History of North America, table 8.1):

1800: 16.0

1810: 16.0

1820: 16.6

1830: 17.3

Based on the above data, the median age in 1790 would likely be just below 16 years old. Unfortunately, almost none of the colonial censuses record full age data. One census that may have was the 1755 Maryland census, but the returns have been lost, and all we have is the summary data, which organized people into age ranges below and above 16 (Wells, Population of the British Colonies in America before 1776; Here are some images of the 1755 Maryland Census, but if you have access to the "17th and 18th Century Burney Collection Newspapers", it was originally printed in June 1764 in the Gentleman's Magazine).