The U.S. Army has reminded its senior commanders to make sure they are not sending anyone under age 18 into combat. The U.S. signed the UN Child Soldier Protocol, which bans the use of soldiers younger than 18, and only allows enlistment of soldiers under 18 with the permission of their parents. The latter rule has long been used by the United States. But before this 1990s agreement, it was common to find 17 year old American soldiers, marines and sailors in combat zones. There were also many cases of underage boys who enlisted using false identification, sometimes with the knowing consent of their parents.
The total number of underage soldiers who died in Vietnam is unknown, but at least five of the troops killed in Vietnam are known to have been under 18 years of age. One, a Marine, seems to have been only 13. There were even more cases during World War II. If found out, such young soldiers were often removed from the combat zone (especially if under age 18).
As it turned out, setting 18 as the age one magically becomes an adult, proved to be a contentious issue when the Child Soldier Protocol was being drafted. Many nations had, for centuries, taken in recruits who were 17, or younger. Most of these nations agreed that, for all practical purposes, there was no difference between 17 and 18 year olds. But the UN bureaucrats had their way, and 18 became the age at which you were no longer a "child soldier."
Nevertheless, it's still popular for 17 years olds (usually recent high school graduates), to enlist in the military. Nearly 10,000 17 year olds do so each year (most for the army) in the United States. It was only two years ago that the U.S. Marine Corps, reluctantly, stopped sending 17 year olds into combat. These were usually 17 year olds only a month or two short of turning 18. They had already finished six months of very tough training, only be told they were "too young" to go off to combat with the guys they trained with.