STOLEN VALOR by BG Burkett states ...average age of U.S. Military in Vietnam War was 22.8 years old. But I won’t quibble with your figure. But the average age certainly wasn’t 18/19 as some here contend. More 20 year olds were KIA than any other age.
Most Americans still believe it a war fought by conscripts, when in fact, 66% of those of us who served were volunteers. Not sure how we change the perception...
http://www.macvsog.cc/vietnam_war_facts_vs_fiction.htm
http://vietnam-veterans.us/popup/facts.html
http://combatveteranonline.net/vietnam014.htm
http://www.vhfcn.org/stat.html