The real median household income of Americans who have completed high school--but have not attained a higher degree--also peaked in the 1970s and has declined since then.
In fact, according to the Census Bureau (Tables H-13 and H-14), the real median household income of an American householder who has completed four years of high school peaked in 1973 at $56,395 in constant 2013 dollars. By 2013, it was down to $40,701. That is a drop of $15,694--or 27.8 percent. (The Census Bureau's Table H-14 publishes the annual median household income from 1960 through 1990 of householders who have "completed" four years of high school. Table H-13 publishes the annual median household income of householders who have 'graduated" from high school or its equivalency from 1991 through 2013.)
According to the BLS’s seasonally adjusted employment numbers, employment in the U.S. manufacturing sector hit a peak of 19,553,000 in June 1979. In April 2015, there were only 12,322,000 employed in the manufacturing sector. That is a decline of 7,231,000—or 37 percent.
The decline has even been greater as a share of the civilian noninstitutional population, which includes all U.S. residents 16 and older who are not on active duty in the military or in an institution such as a prison or nursing home. (This is the population number that forms the foundation of the BLS’s employment numbers.)
In April 1973, the year median household income peaked for householders who have completed high school but not earned a higher degree, there were 146,459,000 in the civilian noninstitutional population and employment in manufacturing was 18,359,000—or 12.5 percent of the civilian noninstitutional population.
In April 2015, there were 250,266,000 in the civilian noninstitutional population and employment in manufacturing was 12,272,000—or 4.9 percent of the civilian population.
In 1973, manufacturing employment as a share of the civilian noninstutional population was 2.5 times what it is today.